Results not as expected, Prop Slip? or something else

[zawoooo]

Hey guys, I've been a long time viewer of these forums but this is my first time posting.

I've recently bought a 4m RIB and a tohatsu 25HP 2Stroke engine and it isn't performing as expected.

A bit of back info

Bought the RIB new and it was packaged with a Parsun 9.9HP 4 stroke engine(9.9x11Pitch). The boat performed surprising well and managed about 23MPH with 2 people on it but wanted a bigger engine for towing.

A mate was selling a 14ft fibreglass with a 25hp tohatsu 2006 model(9.9x13Pitch Prop), so ended up buying that with the aim to take the engine and place it on my RIB. I upped the 25hp to a 30hp with the gasket swap and the fibreglass boat would do about 28-30MPH.

I was super excited to stick the 25HP on my rib with hopes that it would reach about 35MPH as the boat is heaps lighter than the fiberglass. Ended up getting the tohatsu installed last week and was slightly disappointed in the results, as it only got to 26MPH.

The boat seems to do great at 3/4 throttle but that last 1/4 only only gives a few more MPH and what seems like prop slip

I'm thinking that the prop has a huge part to play but i am waiting on a tiny tach to arrive in the mail

Just wondering if anyone here has had a similar experience?

Cheers and thanks in advance, these forums have been a great help

 

[zawoooo]

Re: Results not as expected, Prop Slip? or something else

1 more thing to add.

Both engines are the same weight, although the parsun was a tiler steer and the tohatsu is forward controls so there is an extra 35KG in the console that was installed.

Cheers

 

[pootnic]

Re: Results not as expected, Prop Slip? or something else

It may be the prop,without knowing the rpm,it's kinda hard to say.Someone with the same setup maybe able to help.
Also weight has a big effect on smaller/lighter boats as well as the placement of the weight.
35Kg(-80lbs) is going to to effect you some but I'd still guess closer to 30+ mph.
Is the shaft length the same on the outboards?

 

[zawoooo]

Re: Results not as expected, Prop Slip? or something else

It may be the prop,without knowing the rpm,it's kinda hard to say.Someone with the same setup maybe able to help.
Also weight has a big effect on smaller/lighter boats as well as the placement of the weight.
35Kg(-80lbs) is going to to effect you some but I'd still guess closer to 30+ mph.
Is the shaft length the same on the outboards?

Both are short shaft and are almost exactly the same length.

Have already had a fiddle with building up the transom so the cav plate is inline with the keel but after no noticeable benefit i dropped it back down. Research suggests the correct position is 1-2inces below the keel(it worked for the pasun)

I've been eagerly awaiting for this tach to arrive so i can get a better understanding of whats going on

 

[pootnic]

Re: Results not as expected, Prop Slip? or something else

What works for one motor might not work for the other,especially when one is a 9.9 and the other 25.
The 25/30 will have a bigger prop,gear case and skeg,more in the water ='s more drag.
So it still could be aliitle bit of a setup problem.

For right now(till you get your tach)I don't think I'd go above 3/4 throttle,for long periods.
You might be under propped and already at your rpms,again just guessing.
Is it possible to raise the outboard up abit?easily

 

[steelespike]

Re: Results not as expected, Prop Slip? or something else

Is the rib a ridged hull? is it inflated fully? Some ribs are not conducive to making good speed.
Some have a rather rounded shape and taper oddly at the stern. The shape kind of lets the water cling.
Calculator says rpm , using 10% slip, is about 4500.

 

[steelespike]

Re: Results not as expected, Prop Slip? or something else

Is the rib a ridged hull? is it inflated fully? Some ribs are not conducive to making good speed.
Some have a rather rounded shape and taper oddly at the stern. The shape kind of lets the water cling.
Calculator says rpm , using 10% slip, is about 4500.
Prop store indicates 13" is pretty high up the list with only two props with more pitch.
If the boat has trouble getting clear of the water it might struggle with a 13"

 

[steelespike]

Re: Results not as expected, Prop Slip? or something else

1 more thing to add.

Both engines are the same weight, although the parsun was a tiler steer and the tohatsu is forward controls so there is an extra 35KG in the console that was installed.

Cheers

My info indicates the Parson is 34 kg very roughly 68 lbs. The Tohatsu is 52 kg about 104 lbs.
the console 35 kg about 70lbs. Don't forget the various hardware to control the motor.Bigger gas tank with more fuel? a battery perhaps?
Not a huge amount but some rib,s don't react well to added weight.

 

[Texasmark]

Re: Results not as expected, Prop Slip? or something else

Weight placement is an issue also. If you have a boat with a high rake prop and trim, you can trim the engine in (toward the transom) and get up on plane nicely, all things considered. However when you go to WOT and expect some speed it severely lacks. Also engine rpms are lower than anticipated.

However, when you get on the trim switch, touching nothing else, and start trimming the engine out, the bow lifts our of the water, the boat's wetted area decreases which decreases drag, the speed picks up remarkably as do the engine rpms. Like the boat (a boat) could pick up 10 mph and 1000 rpms in a heartbeat.

Adding your console puts weight forward which increases your wetted area and has a big impact on your speed as I mentioned. Something to think about.

Additionally, lowering the engine on the transom, getting the AV plate below the bottom of the boat increases engine drag which will cost you mph too. Realizing that sometimes you have to lower it to accommodate certain hulls and water conditions, at the lower speeds it doesn't matter all that much.

Mark

 

[zawoooo]

Re: Results not as expected, Prop Slip? or something else

What works for one motor might not work for the other,especially when one is a 9.9 and the other 25.
The 25/30 will have a bigger prop,gear case and skeg,more in the water ='s more drag.
So it still could be aliitle bit of a setup problem.

This is true but very marginal, they are both very similar in size

For right now(till you get your tach)I don't think I'd go above 3/4 throttle,for long periods.
You might be under propped and already at your rpms,again just guessing.
Is it possible to raise the outboard up abit?easily

It is quite easy but the boat went no faster.

Is the rib a ridged hull? is it inflated fully? Some ribs are not conducive to making good speed.
Some have a rather rounded shape and taper oddly at the stern. The shape kind of lets the water cling.
Calculator says rpm , using 10% slip, is about 4500.

Yes its a rigid fibreglass hull and has been inflated to specifications.

Is the rib a ridged hull? is it inflated fully? Some ribs are not conducive to making good speed.
Some have a rather rounded shape and taper oddly at the stern. The shape kind of lets the water cling.
Calculator says rpm , using 10% slip, is about 4500.
Prop store indicates 13" is pretty high up the list with only two props with more pitch.
If the boat has trouble getting clear of the water it might struggle with a 13"

The boat gets clear of the water with ease, it will get out of the hole in a matter of seconds, its just that top end which is letting me down.

My info indicates the Parson is 34 kg very roughly 68 lbs. The Tohatsu is 52 kg about 104 lbs.
the console 35 kg about 70lbs. Don't forget the various hardware to control the motor.Bigger gas tank with more fuel? a battery perhaps?
Not a huge amount but some rib,s don't react well to added weight.

That parsun you are looking at must be a 2 stroke i have a 4 stroke

Weight placement is an issue also. If you have a boat with a high rake prop and trim, you can trim the engine in (toward the transom) and get up on plane nicely, all things considered. However when you go to WOT and expect some speed it severely lacks. Also engine rpms are lower than anticipated.

However, when you get on the trim switch, touching nothing else, and start trimming the engine out, the bow lifts our of the water, the boat's wetted area decreases which decreases drag, the speed picks up remarkably as do the engine rpms. Like the boat (a boat) could pick up 10 mph and 1000 rpms in a heartbeat.

Adding your console puts weight forward which increases your wetted area and has a big impact on your speed as I mentioned. Something to think about.

Additionally, lowering the engine on the transom, getting the AV plate below the bottom of the boat increases engine drag which will cost you mph too. Realizing that sometimes you have to lower it to accommodate certain hulls and water conditions, at the lower speeds it doesn't matter all that much.

Mark

The trim is set so the front of the boat is clear of the water and it bounces along nicely, if i trim it up 1 more tab i get cavitation and loose speed.

I am aware that the engine cav plate being below the keel will produce more drag but then this is how the parsun was setup and i did try raising it to be level with no gain.

As for the console it is actually set quite far back, if i was to dump a passenger and just run me in the boat the weight and distribution would be very similar to the previous setup.

There is definitely something that i need to change as the tohatsu has nearly 3x the power of the parsun
Hopefully this is just a prop issue and a corrected pitched SS prop will fix things up

Thanks for all the replies much appreciated

 

[steelespike]

Re: Results not as expected, Prop Slip? or something else

Checked the weight again this time a 4 stroke overhead valve motor listed at 49 kg. I guess you could call it the same.
By the way 23 from a 9.9 with 2 aboard is impressive.
I found a couple of 25hp Evinrude tests I found the boats intriguing
A 12DL Nautica 410 lbs, one 225 man 5 degree transom wedges and a Hydrofoil, 26 mph.
A Starcraft V16 280 lbs, 2 men 6 gals. gas, battery and gear. 31.6 mph.

 

[steelespike]

Re: Results not as expected, Prop Slip? or something else

delete

 

[pootnic]

Re: Results not as expected, Prop Slip? or something else

I know there's a RIB specific forum(found on google),pretty sure they would be able to help you with real world numbers and what to expect.Good luck,

 

[Outsider]

Re: Results not as expected, Prop Slip? or something else

I am aware that the engine cav plate being below the keel will produce more drag but then this is how the parsun was setup and i did try raising it to be level with no gain.

The only thing similar to the parsun is the boat the Tohatsu is now on. Set your Tohatsu so the cav plate is riding just on the surface of the water when on plane, then decide how to prop it based on measured RPM. Once you have that figured out, adjust the tilt to get the bow angle you desire, knowing full well there are limits to that, too ...