1999 Mariner 125hp 2 stroke autolube

[headlight]

The engine is on a Raider 18ft which is a British built fast fisher weighing about 1,400lbs that my friend bought a couple of months ago.
The engine has only done 120 hours & is in perfect condition.
The prop is a 13.25" Laser 2 with 20" pitch.
We took the boat out this weekend to see how it performed & TBH were a little disappointed.
Acceleration was excellent & the boat topped out at 30.5kts at 5,200rpm.
That would suggest that the boat is correctly propped.
However I have an identical boat with 10hp less & we were expecting at least the same top speed.
My engine is a Honda 115 EFI running a 13.25" x 15" pitch stainless Honda prop.
This gives 36kts at 5,600rpm
Given that the gearbox ratios of both engines are almost identical (2.07 for the Honda & 2.08 for the Mariner) the fact that the Mariner revs 400rpm less flat out should be more than offset by virtue of the much bigger pitched prop I would have thought & we were expecting the Mariner engined boat to be at least as quick as mine if not quicker ?

headlight

 

[drewactual]

Re: 1999 Mariner 125hp 2 stroke autolube

Carb's are finicky.. I'd bet you're running rich on it, which is manifest at the top end where it is more capable of running out of air... that said, and since we're in the prop section, let's look at that alone:

that prop (laser2) is vented, which allows a larger pitch to be spun up similar to one a pitch less... once speed is achieved, surface tension of the water slipping past closes them off, ending the controlled ventilation, and placing the full weight of the pitch on the engine...

to further the difficulty, that blade has a decent rake and it is an aggressively designed progressive blade- meaning the leading edge is likely nearing 18.5p, which is what your engine feels when it comes out of the hole and water is biting on the leading edge.. the trailing edge, in like, is likely 21.5p, which is what the engine feels when the boat is on step and trimmed and the water is slipping off the trailing edge...

you should be seeing a comfortable 5700rpm'ish from that mariner..

the diameter of the laser2, 'on paper', sounds right- as you would want to lessen the diameter while climbing pitch...

I bet if your friend moved down a pitch or two, depending on geometry and features of the prop, you'd see marked improvement- but back to engines: that carb'd mariner is going to struggle to maintain proper air to fuel ratios like an efi can, especially at WOT...

 

[headlight]

Re: 1999 Mariner 125hp 2 stroke autolube

WOT band for the engine is 4,750-5,250 so at 5,200rpm it is basically at the top of this.
The boats are fishing boats so hole shot & top end speed are less important than cruising fuel consumption.
My friend wants to maximize fuel economy as petrol is expensive in the UK.
He is thinking of trying a 22" stainless prop which theoretically will drop max rpm to about 4,800rpm but should allow a set cruise speed with less revs which should equal less fuel ?

headlight

 

[headlight]

Re: 1999 Mariner 125hp 2 stroke autolube

Forgot to mention the engine has recently had a full main dealer service (annual main dealer service from new) & starts on the button, idles fine & pulls up to it"s max revs quickly & smoothly.

headlight

 

[drewactual]

Re: 1999 Mariner 125hp 2 stroke autolube

aye.. I was thinking that engine maxed at 5500~6000rpm... my bad...

the engine needs to have the proper prop no matter the top speed, hole shot, or intended cruise... it's all about load on the engine, not simply RPM's.

in a scenario where two duplicate engines, boats, and loads were negotiating the same seas in the same environments, both at the same RPM, with the same blade geometry except one is pitched 2" higher at both the leading edge and the trailing edge, the speeds would be similar, but the engine with the higher pitched prop is burning considerably more fuel than the engine/boat running a prop selected for it's proper pitch and geometry.

this is more an engine discussion than a prop, however, the prop is the leverage that allows the engine to do what it needs to do. An engine that is maintaining its spool with ease can settle back and sip only the fuel it needs to maintain that speed, whereas an engine that is struggling to maintain a speed will always be pulling more fuel than it needs, as it is trying to increase it's RPM due to added load.... this is why your engine sucks a LOT more fuel at 2000 RPM off plane (displacing water) than it does even at 3000RPM on plane- there is less load.

less load = lower fuel consumption
lower pitched prop = less load
lower load due to prop = less top end, faster engine speeds
being able to achieve higher engine speeds indicates less load on engine, which we always measure at WOT (to match the manufacturers recommendations), which is more easily achieved using a lower pitched prop but at the cost of top end speed...

if you're wanting to increase economy and not worried about top end, 'lower your pitch', don't increase it. You'll find you're out of the hole and on step at a much lower RPM, and capable of holding plane at a lower RPM- and you'll find 200~300rpm over the minimum speed needed to hold plane will be the sweet spot for economy and increasing range of a given tank.

edited to add: IN theory ONLY; does higher pitch equate to faster speeds/more distance covered based on RPM.. for every inch gained in forward movement per revolution, there is a corresponding reduction of applied torque- it becomes harder for the engine to turn the screw with each depletion of available pounds of torque.. if you remove the load on the engine via leverage (or bite into the water), you may find the boat runs faster even with the lesser pitched prop.

 

[headlight]

Re: 1999 Mariner 125hp 2 stroke autolube

aye.. I was thinking that engine maxed at 5500~6000rpm... my bad...

the engine needs to have the proper prop no matter the top speed, hole shot, or intended cruise... it's all about load on the engine, not simply RPM's.

in a scenario where two duplicate engines, boats, and loads were negotiating the same seas in the same environments, both at the same RPM, with the same blade geometry except one is pitched 2" higher at both the leading edge and the trailing edge, the speeds would be similar, but the engine with the higher pitched prop is burning considerably more fuel than the engine/boat running a prop selected for it's proper pitch and geometry.

this is more an engine discussion than a prop, however, the prop is the leverage that allows the engine to do what it needs to do. An engine that is maintaining its spool with ease can settle back and sip only the fuel it needs to maintain that speed, whereas an engine that is struggling to maintain a speed will always be pulling more fuel than it needs, as it is trying to increase it's RPM due to added load.... this is why your engine sucks a LOT more fuel at 2000 RPM off plane (displacing water) than it does even at 3000RPM on plane- there is less load.

less load = lower fuel consumption
lower pitched prop = less load
lower load due to prop = less top end, faster engine speeds
being able to achieve higher engine speeds indicates less load on engine, which we always measure at WOT (to match the manufacturers recommendations), which is more easily achieved using a lower pitched prop but at the cost of top end speed...

if you're wanting to increase economy and not worried about top end, 'lower your pitch', don't increase it. You'll find you're out of the hole and on step at a much lower RPM, and capable of holding plane at a lower RPM- and you'll find 200~300rpm over the minimum speed needed to hold plane will be the sweet spot for economy and increasing range of a given tank.

edited to add: IN theory ONLY; does higher pitch equate to faster speeds/more distance covered based on RPM.. for every inch gained in forward movement per revolution, there is a corresponding reduction of applied torque- it becomes harder for the engine to turn the screw with each depletion of available pounds of torque.. if you remove the load on the engine via leverage (or bite into the water), you may find the boat runs faster even with the lesser pitched prop.

Not disputing anything you say but do you have any idea why of the 2 identical hulls that mine weighing probably 250lbs heavier with 10hp less & spinning a 13.25"x15" stainless 3 blade prop at 5,600rpm is 6-7kts faster than my friends spinning a 13.25"x20" prop at 5,200rpm.
Theoretically on paper my max speed would be 40kts (I get 36-37kts) & my friends on paper is 42kts of which he gets 30kts ?
As to making the engine work harder with a higher pitched prop therefore using more fuel I broadly agree with what you say but these are light hulls (1,400lbs) & are generally run with 90hp motors.
I don"t think that the motor would be struggling maintaining say 20-22kts cruise with a 22" prop.

headlight

 

[drewactual]

Re: 1999 Mariner 125hp 2 stroke autolube

you're not slipping much... you're biting in at around a 97% efficiency.. 3% slip...
he's slipping his tail off, and only getting about 70~75% efficiency.... 25~30% slip...

equating it to tires, (I think you fellers say tyres, no? ), he's delivering torque the bite of the prop can't handle, and it's 'spinning out', and breaking traction (bite into the water in this case)... you're not.

I bet if you were using a 20p prop with no vents and less rake than that laser2, it would be hella more evident, and manifest in more registrable places (such as RPM).

 

[headlight]

Re: 1999 Mariner 125hp 2 stroke autolube

you're not slipping much... you're biting in at around a 97% efficiency.. 3% slip...
he's slipping his tail off, and only getting about 70~75% efficiency.... 25~30% slip...

equating it to tires, (I think you fellers say tyres, no? ), he's delivering torque the bite of the prop can't handle, and it's 'spinning out', and breaking traction (bite into the water in this case)... you're not.

I bet if you were using a 20p prop with no vents and less rake than that laser2, it would be hella more evident, and manifest in more registrable places (such as RPM).

So to improve consumption at cruise do you think he should be going for a higher or lower pitch prop ?
We have a 23" pitch x 13" ally prop to try & if this does what we want he is thinking of purchasing something like this 3 Blade Stainless Steel New Saturn NS3 D series - 50-150 hp PROPELLERS approx 12.5" to 14" diameter with 4.1/4" gearcase

headlight

 

[drewactual]

Re: 1999 Mariner 125hp 2 stroke autolube

If your friend isn't worried about top end speed, at all, he'll find more usable power in a lower pitch... He'll take a lot of load off the engine, he'll jump on step quicker, and stay on step at lower speeds (while continuing to keep load off engine)- he'll turn a higher RPM per MPH/KtsPH, but the RPM will be 'cleaner' (not as much excess fuel being introduced per cycle that isn't being fully burned)..

yes- to improve consumption at cruise, a lower pitch will benefit him... not crazy lower than he's using, but lower.


about the prop linked:

the prop you suggested isn't vented- that laser2 is.. pitch matching pitch, a vented prop spools up faster and more efficiently than a non-vented to a degree matched by pitch... another way of saying it: a 21p vented will achieve a higher RPM quicker than a 21p unvented, or about the same as a 20p unvented would..

I can't see anything about the progressive pitch geometry (I really wish maker would express that angle in their advertising instead of making us experiment)... I can see that they are tuliped blades with a lot of blade surface- which is good for sustaining a load in motion..

the picture angle of the prop doesn't allow the rake to be visible- which plays into how quickly it spools up as well, and how it shifts the water flow from the leading edge to the trailing edge as speed is achieved. that is manifest in bow lift.. heavier rakes lift more bow.. this comes into play at low speed planing- you'd want your boats attitude in the water fairly flat for line of sight, but not so much it isn't riding on the pad but plowing instead- a moderate rake is best... simply because it omits the data about rake, I'd guess it is average, whiich is good for your purposes..

it is stainless, which means little to no flex- which means a 20p aluminum or composite prop runs about the same as a 19p under torque.. to match between the two materials (and matching geometry and features of the prop) you'd have to use a 19p s/s for a fair comparison to aluminum or composite....

long story short: I'd use a 18p s/s, but that isn't available- so I'd err on the side of under-propping, with the 17p, which is available, if you're dead set on that selection. Notice the 17p increases diameter slightly, or by a 1/4 inch... that is a good thing, while moving down in pitch, to keep the engine from over-revving and to gain even more bite.

 

[headlight]

Re: 1999 Mariner 125hp 2 stroke autolube

We will try the 13" x 23" ally this weekend just to see what happens as we have it & I will post the results.
I am guessing that max revs will drop to about 4,700rpm

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[steelespike]

Re: 1999 Mariner 125hp 2 stroke autolube

We will try the 13" x 23" ally this weekend just to see what happens as we have it & I will post the results.
I am guessing that max revs will drop to about 4,700rpm


headlight

And the slip may go up.

 

[headlight]

Re: 1999 Mariner 125hp 2 stroke autolube

I suppose the other possiblity is that the tacho is reading incorrectly & the engine is not actually revving at 5,200rpm WOT ?
We have noticed that sometimes the tacho reads zero at idle but seems to work fine as soon as the revs are increased.
We will check out the tacho this weekend as well.
I spoke to 2 prop specialists here in the UK yesterday.
One advised a 22" pitch stainless 3 blade & the other the same or a 21" pitch 4 blade stainless ?

headlight

 

[drewactual]

Re: 1999 Mariner 125hp 2 stroke autolube

matching the prop to the engine for all around performance would lend credit to your prop specialists, and that ONLY after considering your gear case ratio..... however, the boat isn't a light one for that engine, AND you've indicated your friend is more interested in extending cruise range, and then I interjected that using a lower pitch would take a lot of load off the engine for that purpose.. Your prop specialist doesn't sound like he's taking your specific needs into account, and is only matching the prop to the engine...

I think 22 stainless steel is over propping, no matter the circumstances for that rig. 21p in four blade is about the same as the 22p three, so far as the effort to turn it is concerned, and though it will increase bite, it is still over propping by my reckoning.

I still think the better selection for your friend is a 18p stainless, or a 19p aluminum, but the one you indicated interest in doesn't come in a 18p, so I would err on the side of safety (so far as the motor is concerned) and select a 17p in that model- OR, (and the better option) I would continue to search around for a 18p ss 13.25 diameter with a decent rake, good cupping, and progressive pitch-

realize, again, an aggressive pitched prop gives the average pitch, not the mid blade pitch.. one rated at an 18p could be 16.5p on the leading edge and 19.5p on the trailing edge, or it could be 17p on the leading edge and 20.5p on the trailing edge.. <- that is absolutely prop geometry dependent and pretty much the reason we all have so much trouble finding the perfect prop...

I would consider a 19p~20p s/s ONLY IF it was vented...

again, this is to tailor to the expectations you've stated your friend is most interested in.. if he was matching for best overall performance, I'd consider no bigger than a 20p unvented, and a 21p vented in stainless steel, (and that may be right on the line of over-propping) and a 21p unvented or 22p vented n aluminum, as it will flex to near a pitch less anyway.

best of luck!!

 

[headlight]

Re: 1999 Mariner 125hp 2 stroke autolube

matching the prop to the engine for all around performance would lend credit to your prop specialists, and that ONLY after considering your gear case ratio..... however, the boat isn't a light one for that engine, AND you've indicated your friend is more interested in extending cruise range, and then I interjected that using a lower pitch would take a lot of load off the engine for that purpose.. Your prop specialist doesn't sound like he's taking your specific needs into account, and is only matching the prop to the engine...

I think 22 stainless steel is over propping, no matter the circumstances for that rig. 21p in four blade is about the same as the 22p three, so far as the effort to turn it is concerned, and though it will increase bite, it is still over propping by my reckoning.

I still think the better selection for your friend is a 18p stainless, or a 19p aluminum, but the one you indicated interest in doesn't come in a 18p, so I would err on the side of safety (so far as the motor is concerned) and select a 17p in that model- OR, (and the better option) I would continue to search around for a 18p ss 13.25 diameter with a decent rake, good cupping, and progressive pitch-

realize, again, an aggressive pitched prop gives the average pitch, not the mid blade pitch.. one rated at an 18p could be 16.5p on the leading edge and 19.5p on the trailing edge, or it could be 17p on the leading edge and 20.5p on the trailing edge.. <- that is absolutely prop geometry dependent and pretty much the reason we all have so much trouble finding the perfect prop...

I would consider a 19p~20p s/s ONLY IF it was vented...

again, this is to tailor to the expectations you've stated your friend is most interested in.. if he was matching for best overall performance, I'd consider no bigger than a 20p unvented, and a 21p vented in stainless steel, (and that may be right on the line of over-propping) and a 21p unvented or 22p vented n aluminum, as it will flex to near a pitch less anyway.

best of luck!!

Thanks for the advice Drew, I am not ignoring it honest.
We are trying to prop this boat for maximum fuel consumption at a cruise speed of say 20-22kts.
The boat weighs about 650 kgs or 1,400lbs.
Here in the UK 125hp for that weight of boat is considered powerful (we don"t tend to run anywhere near as high hp outboards as you do in the states, partly I expect due to our much dearer fuel costs) & plenty of our boats of around 2,000lbs are only rated to a max hp of 90 & 3,000lbs boats only rated to 115hp.
With his boat weight & hp we were expecting a top speed of around 36-38kts.
I think we have managed to get our hands on a standard Mercury 3 blade ally prop of 13.25" x 21" pitch to try & I think that that will at least give us a chance of making some educated guesses of what to finally fit when compared against the present Laser2 performance.

headlight

 

[drewactual]

Re: 1999 Mariner 125hp 2 stroke autolube

more powerfully equipped doesn't exactly equal more fuel consumption... it just means it's there if you need/want it... most of us use it far more often than we need it.... but....

An engine that produces requested power without excess load on the engine is going to sip less fuel than an engine under heavy load, and the displacement differential is often negated... boats are peculiar in that they need a lot more power displacing water (under plane) than they need on plane- the trick is getting them on plane and trimming to a condition where the motor isn't struggling to maintain the hulls plane... that equals best fuel economy...

you can approach it two ways, at least.. you can either use the engines capacity to produce energy, or you can use gears to create required leverage to do the same... it seems 'our' market tweaks the power angle, and yours tweaks the leverage.. either works...

'we' can punch up and on plane using higher gears (lower numerically, manifest in this discussion in terms not only of gear ratio, but of prop pitch as well) because we have a tendency to overpower our vessels... based on what you've stated, and that 125 is heavily powered for a vessel up to 3k#, it certainly sounds like you guys lean more towards leverage... like I said, either works, but, as astonishing as it sounds, apples-to-apples, and speaking of the same boat with the exception of engine and screw, I'd be willing to drop coin that a vessel which is 125hp and over propped will burn nearing if not exceeding that of a 175hp properly propped duplicate vessel in the few seconds it takes to push up on plane.. to further that notion, the lessor powered over propped boat is loading the engine to hold plane at cruising speeds, though the over powered properly propped vessel is sipping away it little load to do the same, but going faster..

the range of these two vessels are likely near the same... the dude with the over powered properly propped vessel is finishing his sandwich and tossing his last beer by the time the underpowered over propped feller arrives, though..

if your interests lie in fuel economy and range, and no matter if you're here or there- over powered or under powered, the prop is the angle of attack (no pun intended)- UNDER propping (using a pitch or two less than required to easily achieve maximum engine RPM range) is the ticket- you get on plane easier, and you stay on plane easier.. the RPMs are higher, but RPM's only tell part of the story about an engines consumption- the load tells the entire story.. lessen the load 25% and increase the RPM's 25%, and you have a break even... lessen the load 25% and increase the RPMs >25%, and you've increased your economy, and range.... if your engine/gear case/prop combo allows you to do that, you're achieving the results you've indicated desirable.

it's an interesting subject about props... I don't know how many folks have truly thought it through- but I understand why most haven't- and it's because there are a ton of things otherwise to be concerned about- so we use the mathematical formula and the easiest way possible to match, so we can be done with it... it's pretty simple: "is WOT within engine manufactures recommended range? Does it go fast enough? Does it 'come out of the hole' within reason? GOOD, I'm done.."... but there is more that can be accomplished by going deeper into it, but each depth is going to give something away to gain something else. In your case, and based off what you've shared, you'll likely be better served by under propping a pitch or two.

 

[hwsiii]

Re: 1999 Mariner 125hp 2 stroke autolube

headlight, sorry to butt into your posts, but by just doing a quick perusal of the data you have provided, I would say that your setups on your boats are off from where they should be, or there is something wrong with the data that you are providing.

1. Your partners boat is showing a prop slip of 26%. Which is just waaay too high.
2 Your boat shows a prop slip of -8%. And that is not realistic, even though prop manufacturers don't always tell the correct pitches for their props. About the BEST prop slip that can be attained with any prop is about 7% to 8%, according to the manufacturers pitch that he says. So that means that the numbers are off by about 15%.

I have written prop programs that I use to help people find the right prop for their boat, and then it becomes an art instead of mathematics and you have to understand the differences in prop geometries to get a final prop that is a good fit for their particular application.

You also are trying to compare your different props to each others mainly by pitch, and it doesn't work that way. Blade geometry makes all of the difference in the world.

And exchanging props for lower RPM's to gain fuel efficiency hardly ever works, it usually just lugs the engine and increases wear and tear.

If the numbers you have given above are anywhere close to right you could improve your partners boat considerably by just getting a 4 blade aluminum prop. His slip is waaaaay higher than it should be. If the problem is not the hull or motor itself.

I have helped people prop a few boats on here and I usually get it right the first time, according to what I tell them to expect with the prop I recommend.

I do believe that your partners boat definitely needs to have a prop change.

Throwing money at a problem is not always the best way to correct it.

Please, don't take anything I have said in this post is derogatory, as it is not meant to be, I just really think that y'all are going in the wrong directions with your theories on the problems.

H

 

[headlight]

Re: 1999 Mariner 125hp 2 stroke autolube

Firstly thanks to all for their interest in this thread.
Yesterday we swapped props on my friends boat.
Off came the 20" Laser2 & on went a standard Mercury 13.25x21" ally 3 blade.
Results are that WOT has dropped from 5,200rpm to 5,000rpm but top speed has gone up a knot to 31kts.
Cruise rpms have changed from 20kts at 3,800rpm to 21kts at 3,400rpm.
Acceleration is fractionally slower but still extremely quick.
I appreciate that lower rpm for a given speed does neccessarily equal better mpg.
Top speed though is still low as I know 8 maybe 10 people running the same hull with 90-115hp that are getting faster top speeds with good consumption running props that rev into the engine makers WOT band.
We have no idea what consumption is like yet as yesterdays tests were carried out in a lagoon & only consisted of of a few miles at WOT.
We will use the boat fishing for a couple of trips & see what consumption is actually like then decide where we go prop wise.
One thing that was apparent from yesterdays test is that there is scope for raising the engine up on the transom as at WOT you can only just make out the plate above the AV plate & not the AV plate itself.
We will see what mpg is like & decide then whether to just fit a stainless prop or lift the motor & test some more.
Finally can someone explain how they work out that I am getting negative slip on my set up please as my figures give a theoretical top speed of 40kts & a real speed of 36kts which would suggest to me that my slip is about 10%.
My friend Glyns theoretical top speed according to my calcs is 42kts & he gets 30kts (31kts now) which gave a slip of about 28% which agrees with others figures ?

headlight

 

[SkiDad]

Re: 1999 Mariner 125hp 2 stroke autolube

If that was my boat I'd be running a 17 or 18 pitch on it - i don't think you have any concerns hitting 5600-5800 rpm on a 2 cycle - when I was a kid guys ran em higher than that even. I think you will find better ecomomy with a lower pitch prop - it may surprise you. And you should at least hit 35 knots with your setup.

 

[hwsiii]

Re: 1999 Mariner 125hp 2 stroke autolube

Headlight, you evidently did not use the same formulas, or you used different data to come up with your speed and his speed, or otherwise you should have come up with numbers more like this. Now, if your prop was actually a higher pitch than what you say it is that would make a lot more sense in this whole conversation.

http://s369.photobucket.com/user/hwsiii3/media/Headlight.png.html?filters[user]=82584274&filters[recent]=1&sort=1&o=0

http://s369.photobucket.com/user/hwsiii3/media/PropSlip.png.html?filters[user]=82584274&filters[recent]=1&sort=1&o=0

headlight " I suppose the other possiblity is that the tacho is reading incorrectly & the engine is not actually revving at 5,200rpm WOT ? We have noticed that sometimes the tacho reads zero at idle but seems to work fine as soon as the revs are increased.
We will check out the tacho this weekend as well."

My best guess is that you will find his tach is not right and he is using a prop that has too much pitch and/or blade geometry for his boat and motor combination, and his motor needs to be raised up higher on the transom, by at least one notch and probably two.

A classic way to tell if a motor is way too low on the transom is to look at the motor when it is running at higher planing speeds and if you see a lot of water spray coming from both sides of the motor leg and out to the sides you need to raise the motor up at least two notches.

Having a motor way too low on the transom will slow a boat down tremendously and lower RPM's dramatically, believe it or not.


H

 

[headlight]

Re: 1999 Mariner 125hp 2 stroke autolube

Headlight, you evidently did not use the same formulas, or you used different data to come up with your speed and his speed, or otherwise you should have come up with numbers more like this. Now, if your prop was actually a higher pitch than what you say it is that would make a lot more sense in this whole conversation.

http://s369.photobucket.com/user/hwsiii3/media/Headlight.png.html?filters[user]=82584274&filters[recent]=1&sort=1&o=0

http://s369.photobucket.com/user/hwsiii3/media/PropSlip.png.html?filters[user]=82584274&filters[recent]=1&sort=1&o=0

headlight " I suppose the other possiblity is that the tacho is reading incorrectly & the engine is not actually revving at 5,200rpm WOT ? We have noticed that sometimes the tacho reads zero at idle but seems to work fine as soon as the revs are increased.
We will check out the tacho this weekend as well."

My best guess is that you will find his tach is not right and he is using a prop that has too much pitch and/or blade geometry for his boat and motor combination, and his motor needs to be raised up higher on the transom, by at least one notch and probably two.

A classic way to tell if a motor is way too low on the transom is to look at the motor when it is running at higher planing speeds and if you see a lot of water spray coming from both sides of the motor leg and out to the sides you need to raise the motor up at least two notches.

Having a motor way too low on the transom will slow a boat down tremendously and lower RPM's dramatically, believe it or not.


H

Tacho was checked by connecting a Tiny Tach temporarily & although the boats tach sometimes reads zero at idle once moving the 2 tachs matched rpm.
As I said there is definitely scope to raise the motor as there is some spray coming from either side of the leg.
What is surprising is the fact that other people with the same hull & less hp with long shaft ("20") motors mounted on the transom the same as my friends get a higher top speed ?


headlight