swapped prop for same - altered height - revs through the roof

[5teve]

Hi guys

Strange one here... Aluminium 19ft cabin boat, 140 tohatsu 2 st - all speeds measured are gps

After smashing the original tohatsu prop and only ever getting to 5200-5300rpm WOT I decided to take a look at the variables, do some testing and get all my figures straight before getting a stainless 4 blader (almost all out boat use is ocean in up to 10ft swells).

Firstly I ordered a second hand prop made by yammy in the same pitch as the tohatsu 13 1/4 x 17. While I was waiting for this to turn up I took a much closer look at engine height. Measuring gave me the ventilation plate sitting around 2 inches below the hull, which would explain the limited WOT results. ( I dropped it down 2 holes when I got it because of ventilation, But more like my newness to boating). So I raised the motor back up so the ventilation plate is almost level with the bottom.

New prop arrived, paint sanded off as it looked pretty rough, fitted it, out on the water for some testing. Holy hell it has some holeshot, it jumps out of the water. But now the problem.... I cannot get to WOT ....at half throttle and neutral trim I'm pushing 5900rpm and around 28knots. The old prop with lower motor position gave 32 knots at 5200-5300 rpm so I don't know what the hell is going on. I thought same pitch should give same results but there must be something drastically different between the props but I don't know what. I doubt its the raising of the engine 2 holes..

Other than the stamp on the prop is there and easy way of checking actual size. This isn't helping my facts to establish the pitch I need for a 4 blader!

Any thoughts what could be going on here?

Appreciate any help and opinions

Steve (Australia)

 

[limitout]

Re: swapped prop for same - altered height - revs through the roof

you would be surprised how much extra drag and lost speed you have with the motor too low. is it the cause for the difference you see? I think its a big part of it, but im not going to say that's the whole difference.

others who know more will give you more advice on what you need but it sounds like maybe the other prop wasn't the right prop from the start. I suggest you forget about the differences in prop performance and attribute it strictly to the new motor position.

first thoughts are you need to stick with the same type style of prop you have now but go up in pitch 2 sizes so if it is a 17 go to a 19 to bring your rpms lower and your speed back up to where they were before. I think you should find the right prop size before selecting your 4 blade prop but i'll leave that up to the experts.

In the end I think in the final result with the correct prop and correct motor height, you will most likely see better top speed and higher wot rpms because the motor isn't plowing its way thru the water.

 

[steelespike]

Re: swapped prop for same - altered height - revs through the roof

It's usually suggested to make only one change at a time and test.
What is the dry weight of the boat?Is it a full blown cabin boat or a cuddy?
From your performance I would say the replacement isn't a 17" or has a lot less cup.
You really don't know what your getting with used.
Your slip is 14% with the old prop and 32% with the replacement.
Something is amiss.
Typical slip is about 8 to 12%.

 

[Bondo]

Re: swapped prop for same - altered height - revs through the roof

It's usually suggested to make only one change at a time and test.
What is the dry weight of the boat?Is it a full blown cabin boat or a cuddy?
From your performance I would say the replacement isn't a 17" or has a lot less cup.
You really don't know what your getting with used.
Your slip is 14% with the old prop and 32% with the replacement.
Something is amiss.
Typical slip is about 8 to 12%.

Ayuh,.... Agreed,....

'n usin' 2 props from 2 Different manufacturers means they really ain't the same prop,....

Lotsa variables involved with different manufacturers,.... even with the same pitch, 'n diameter,...

 

[phillnjack2]

Re: swapped prop for same - altered height - revs through the roof

the prop you have now sounds about right.
you have a decent holeshot but once the prop is pulling hard the bush is letting go !!!!!!!!!

now take the prop off, clean the back of the prop so you can place a mark on the rubber bush and the metal part of the prop.
make this a definite mark as your going to be looking for prop bush slipping.

take boat out and run it hard, upto 6000 wont hurt it at all for a short burst anyway.
then when you stop take prop off and see if marks have moved, do this a few times, if no change then prop bush is good.
for to have it slip 3 times and land in same place is almost impossible.

also this prop could be one of those that is terrible when run high and suffers cavitation bad..

it sounds like a slipped hub to me.

.

 

[Fed]

Re: swapped prop for same - altered height - revs through the roof

Got an external keel?

 

[phillnjack2]

Re: swapped prop for same - altered height - revs through the roof

good thinking fred

.

 

[5teve]

Re: swapped prop for same - altered height - revs through the roof

It's usually suggested to make only one change at a time and test.
What is the dry weight of the boat?Is it a full blown cabin boat or a cuddy?
From your performance I would say the replacement isn't a 17" or has a lot less cup.
You really don't know what your getting with used.
Your slip is 14% with the old prop and 32% with the replacement.
Something is amiss.
Typical slip is about 8 to 12%.

Dry weight is unknown, (there are very few of these boats) but with the motor and full fuel tank and all the accessories its about 1.2ton (that excludes people) Its a cuddy, but 5mm bottom and 4mm sides so fairly heavily built

14% sounds about right for the old prop, it was pretty tired before i smashed it! 32% is bad.. I assume that is calculated at a gearbox of 2:1 and with the pitch of 17? You are right something is seriously amiss.. how would identify cupping between props?

I know i am only meant to make one change at a time, however I run a business and have very little time, so try to do things in tandem (in an educated way)

It doesnt feel or sound like its ventilating when its running certainly not when accelerating, when turning hard with trim up it ventilates but that is resolved by bringing the trim down.

the prop you have now sounds about right.
you have a decent holeshot but once the prop is pulling hard the bush is letting go !!!!!!!!!

now take the prop off, clean the back of the prop so you can place a mark on the rubber bush and the metal part of the prop.
make this a definite mark as your going to be looking for prop bush slipping.

take boat out and run it hard, upto 6000 wont hurt it at all for a short burst anyway.
then when you stop take prop off and see if marks have moved, do this a few times, if no change then prop bush is good.
for to have it slip 3 times and land in same place is almost impossible.

also this prop could be one of those that is terrible when run high and suffers cavitation bad..

it sounds like a slipped hub to me.

.

I'll check this out thanks.. I have the boat in the marina at the moment (having trailer re-galved) so will need to be careful how i do this or wait until its out of the water again (end of next week)

Problem we have here is the weather. Its hard to WOT tests in big seas especially in an ally without losing your teeth I may be able to do some next weekend when we go to a big inland estuary, but if the wind is blowing its as bad as the ocean!

I'm probably leaning towards a crappy prop..

Got an external keel?

By external keel, I assume you are referring to the 'spine' that runs down the bottom of the boat (about an inch below where the bottom sheets are welded to it) its an upside down T until about a foot from the back where it becomes just a vertical plate about 3/8" thick. the cross piece that makes the T is about an inch wide also but rounded on its external face

Motor in neutral trim sits a fair way back off the transom as there is a 15deg slope on the transom.

Are you thinking bad ventilation? or something else?

Thanks for the thoughts so far guys.. appreciated.

Steve

 

[Fed]

Re: swapped prop for same - altered height - revs through the roof

Are you thinking bad ventilation?

Yes.

My mistake Steve, I should have said prop slip caused by bubbles from the external keel, lazy on my part it was easier to quote you & say 'Yes'.

Just recently I had a tiny Y shaped stick about 6" long by 1/4" diameter hooked on my lower unit right above the bullet, that was all it took.

 

[5teve]

Re: swapped prop for same - altered height - revs through the roof

Yes.

From the keel?

The engine / prop really doesn't sound / feel like its suffering with ventilation. hit around 3500 rpm and hold it.. then nail the throttle and the boat jumps, i would have thought with ventilation i'd of had slippage and it would have gradually increased speed? I'm open to more experienced thoughts on that tho!

Ah just noticed your aus based too fed.. hence you knowing the boat.. its a 575 stacer ocean runner. I'm beginning to think that the prop is just crap..

The engine used to be at this height when i first bough the boat (with the original tohatsu prop) with no issues with over revving, just mild ventilation on sharp turns, hence my likely newbie mistake lowering the engine.

For our american friends - I don't want to go buying props left right and centre because of the horrendous cost here. To give you an idea a 4 blade St t solas is around $800+ and aluminiums are $300+ hence the used prop i 'thought' would do the job of testing! Once the testing is done i'll be buying from overseas. We also don't have the generous return policies that you guys enjoy there!

Thanks

Steve

 

[steelespike]

Re: swapped prop for same - altered height - revs through the roof

I used 2.00 ratio and 17" pitch.
If the hub holds for hole shot it isn't likely to be bad.
Once you establish if the hub is sound you may be able to have a prop shop add a little pitch or cup to the prop it appears it needs some cup.
The Yamaha prop may be by Solas;I've seen it mentioned in various posts.
Its normal to trim down for turns.
Adding cup may eliminate venting in turns and get your rpm under control.

 

[5teve]

Re: swapped prop for same - altered height - revs through the roof

I used 2.00 ratio and 17" pitch.
If the hub holds for hole shot it isn't likely to be bad.
Once you establish if the hub is sound you may be able to have a prop shop add a little pitch or cup to the prop it appears it needs some cup.
The Yamaha prop may be by Solas;I've seen it mentioned in various posts.
Its normal to trim down for turns.
Adding cup may eliminate venting in turns and get your rpm under control.

Thanks Steelspike

And there in lies another issue.. Not sure we have anything like a prop shop around here. Labour costs would be so high for alterations I would be better off ordering new props from Iboats!

Once we get a bit less wind and swell, i'll do more testing and get some accurate figures. flat water is a rare thing here. Doubt I'll see it for another week or so and then only a slight chance i'll see it on inland waters (think chicago!) here is our next weeks weather! Perth Weather Forecast (Swell, Wind, Tide, Rain & Temperature) we are on the west coast of Aus and southerly isnt good. swell is staying fairly low tho.

The yammie prop is an oldish prop. it has a K on the end of the part number and the part number sits on the face nearest the thrust washer. I cant say that anything seemed unusual when compared to the tohatsu prop.

I'm not looking to modify the prop in any form as its not a keeper, just a tester. Once I get the figures i'll be buying a 4 blader but until i get the correct WOT figures to get a baseline then I dont know whether to stick with 17" pitch, drop to 15" or go to 19! Looks like i may be looking to get a new prop again of the same size to verify the pitch. Unless that is that there is an easy way of measuring pitch?

Thanks

Steve

 

[dan02gt]

Re: swapped prop for same - altered height - revs through the roof

Steve,

Have you tried dropping the motor back down?

I've spent a fair amount of time setting up bass boats with jack plates that allow you to easily adjust the motor height. I can tell you from experience that 1/2" in motor height can be all the difference in the world. I've see 1/2" cost 6 MPH and gain 500 RPMs due to excessive prop slip. Slip doesn't sound like ventilation unless it's really really bad. Most of the time everything sounds OK except for poor performance numbers.

Also different props need to run at different heights to perform the best. For instance my 21P Mercury Tempest would great 1" higher than my 21P Mercury Laser II. Raise the Laser II that high and it would slip like crazy. On the other hand lower the Tempest down to where the Laser II liked to be and it would loose 2 MPH. Things like cup, rake, blade geometry, and diameter all effect optimal motor height.

 

[Fed]

Re: swapped prop for same - altered height - revs through the roof

Steve I just got in a knot quoting, editing, posting etc.
Re-read my post #9 I have changed it slightly.

 

[5teve]

Re: swapped prop for same - altered height - revs through the roof

Steve I just got in a knot quoting, editing, posting etc.
Re-read my post #9 I have changed it slightly.

Thanks Fed - Appreciate the update. Makes sense what you say, I'll take some pictures of the area and levels etc when I get it back out of the water. Trailer is currently in the galvanisers and the boat is in a local marina pen.

Steve,

Have you tried dropping the motor back down?

I've spent a fair amount of time setting up bass boats with jack plates that allow you to easily adjust the motor height. I can tell you from experience that 1/2" in motor height can be all the difference in the world. I've see 1/2" cost 6 MPH and gain 500 RPMs due to excessive prop slip. Slip doesn't sound like ventilation unless it's really really bad. Most of the time everything sounds OK except for poor performance numbers.

Also different props need to run at different heights to perform the best. For instance my 21P Mercury Tempest would great 1" higher than my 21P Mercury Laser II. Raise the Laser II that high and it would slip like crazy. On the other hand lower the Tempest down to where the Laser II liked to be and it would loose 2 MPH. Things like cup, rake, blade geometry, and diameter all effect optimal motor height.

Thanks Dan.. its certainly a complicated field! As I just mentioned above I havent done anything with the motor (just a single run in rough weather) as the boat is in the water and I have no way of getting it out until the trailer is back and together! I would dearly love to have a jack plate to let me do that.. but again its $$ i cannot afford right now (1 year old business it taking most of it) I have thought about a fixed plate with a small amount of setback however but again.. thats a while away.

I'll get the boat back out of the water, then take it down to the inland estuary next weekend and then do some real testing. I'll also get some piccies to show whats going on.

Out of interest, when we first got the boat, we didnt have these issues with the tohatsu prop other than ventilation on turns (i think my newness to boating didn't help) however the boat spent its first couple of years permanently in the water so was antifouled. Now the antifoul is getting a bit flaky.. i'm pondering over if that could cause an issue especially if its antifoul flaking on the keel? I keep meaning to look at having it taken off, but it seems a huge job

will update you, when I have more info... thanks for your help so far!

steve

 

[limitout]

Re: swapped prop for same - altered height - revs through the roof

so before you ever needed a prop you had a little blowout or ventilation problem way back when you first got the boat and you thought it was a good idea to lower the motor down 2 positions lower to solve that issue and it worked out ok even though the speed and rpms aren't where they should have been.

moving 2 holes at a time is way too much of an adjustment so try it at only one hole different and see how the rpms are with that setting after you put the correct original prop back on it. (keep that yammy for an emergency spare or sell it)

what you had before was since you lowered the motor 2 notches you had the motor too deep so it was plowing through the water creating drag and its not going to be very efficient setup that way even though you can always find the right prop combination to make things work even with an improperly adjusted motor.

the motor needs to be as high up out of the water as it can go without causing blowout or ventilation issues

 

[5teve]

Re: swapped prop for same - altered height - revs through the roof

so before you ever needed a prop you had a little blowout or ventilation problem way back when you first got the boat and you thought it was a good idea to lower the motor down 2 positions lower to solve that issue and it worked out ok even though the speed and rpms aren't where they should have been. - As I said... I was new to boating, did a little light reading and came to my own (maybe wrong) conclusions the boat had been run with the motor height set as it is now for 3 years. :joyous:

moving 2 holes at a time is way too much of an adjustment so try it at only one hole different and see how the rpms are with that setting after you put the correct original prop back on it. (keep that yammy for an emergency spare or sell it) - Cannot put original prop back on as it had an argument with a big rock; half a blade is missing! This is why i bought the yammie one, A) to do the testing and B) to keep as a spare once I had the right prop. I thought moving the motor back up would be returning it to the 'factory' setup which would enable my testing to be a little more productive, rather that dragging the motor around buried too deep

what you had before was since you lowered the motor 2 notches you had the motor too deep so it was plowing through the water creating drag and its not going to be very efficient setup that way even though you can always find the right prop combination to make things work even with an improperly adjusted motor.

the motor needs to be as high up out of the water as it can go without causing blowout or ventilation issues

HI Limitout

I have answered a few points above in red. Thanks for the info.

I have had another thought. Cupping! The tohatsu prop is from what i can gather cupped. the part number is 48 77344 t45 17p it feels like there is a definite curve on the trailing edge. It may have been added after.

The yammie however is fairly old, just has 13 1/4 17p K on the back (where the thrust washer is) and from what i can see (need to confirm) doesnt appear to have any kind of cupping on the trailing edge. Could this explain the significant difference?

I have found someone in Perth that can do pitch measurement and add cupping, so will be calling them later to see how much they charge for checking and modifying props.

Could the cupping make such a large difference?

Steve

 

[jestor68]

Re: swapped prop for same - altered height - revs through the roof

Could the cupping make such a large difference?

Steve[/QUOTE] Yes; cupping adds effective pitch( 1-2 inches ).

 

[Sea Rider]

Re: swapped prop for same - altered height - revs through the roof

If wanting to achieve the best power out of that 140 Tohatsu, with well ballanced boat, and cav plate riding paralell to water surface on plane, water flow should pass slightly under small upper water deflector plate. If it's a long shaft water flow is probably hitting the round non edge upper portion of middle leg, you could have unwanted water splashes at back transom which will slow you down a bit.

The ideal tail height must be visually checked by a transom spotter, then dial best transom/engine height for that engine, play with height positions untill opt leg height is reached, then you can play with all prop pitches you like.

Happy Boating

 

[limitout]

Re: swapped prop for same - altered height - revs through the roof

HI Limitout

I have answered a few points above in red. Thanks for the info.

well I understood your prop was destroyed but the idea I was saying was to buy the listed exact replacement tohatsu prop for your motor and not use the Yamaha which isn't the correct brand AFAIK and may give you a false impression of the results. I think you may have had the correct prop all along (or close to it) but you just had the motor set too low for best performance.

I am of the understanding that different brand props use different rake and cupping designs to match the power and gear ratios they use so performance can vary in a dramatic way between them.

as to cupping, yes cupping is a very big deal and is as important as pitch and diameter.

as to motor height, my intention was to guide you to return the motor to the factory position as best as you can (cav plate even with or just above the bottom of the hull) and then and only then decide based on the way the correct prop (the new tohatsu prop I suggest you buy) works, and then decide based on how it performs, if you have the right prop or not.

if you continue to get blow outs and the test results suggest you do need to lower the motor position, you should only move one hole at a time and never two because that is way too much of a difference.