Dingy propeller pitch query

[markcouz]

I am looking for some input which I hope will save me some boat bucks if I make the wrong choice.

I have a 3.2m dingy (glass v-hull) with Tohatsu MFS9.8B 4 stroke long shaft engine + doelfins already added. The dingy jumps up on the plane and rides beautifully when i am alone (+- 110kg) but chugs along in the hole when I add my wife (+- 50kg).

I can get it onto the plane with both aboard if I scrub the hull and wait for a calm day, so I guess it is close to performing as I would like, but just not there most of the time.

My quick research indicates that reducing the pitch would provide more low end power to get onto the plane at the expense of top end speed, considering that I currently cant get out of the hole this seems to be a minor sacrifice!

I currently have what I believe to be the standard 3 blade prop supplied with the long shaft engine - the only marking is [7.5] pressed into the back of one the the blades, so I believe the prop to be a 3x8.5x7.5 - so my question for those that know...

Should I be going for a 4 blade prop (options: 4x8.5x7 or 4x8.7x5)?
Should I just be dropping pitch (options: 3x8.9x7 or 3x8.5x6)?
Or something else that I haven't found out about yet?

Thanks for taking the time to read!

[motion30]

1st purchase and install a heads or a dofin. One of those plates that extends the cavitation plate. That will be a big help for you. If it were me I would purchase the lowest pitch prop I could find. It might hurt your top speed a little I don't feel that's very important. Those 2 things together should make a big difference

[rslifkin]

The only way to really get it right is to buy one of the small inductive tachometers and install one on the engine. Then you'll know what RPM you're seeing at full throttle with various loads.

If you're just not making enough RPM, you probably want a lower pitch prop (or add trim tabs to the dinghy for more lift to help it plane at a lower speed, or both). If your RPM is good but it's still struggling along and can't get out of the hole then it might be worth going to a 4 blade to get more bite (the 7 pitch 4 blade would likely be a good pick in this situation). Solas makes props for that outboard that give a few pitch options, etc. that aren't available in the OE props (and at least on the 6hp, some people have found the Solas props to turn more RPM and run faster than the same pitch/diameter OE Tohatsu prop).

Play with your trim position on the outboard as well as the mounting height (ideally you want it mounted high enough that the anti-vent plate isn't too far below the water surface while on plane lightly loaded). Having the outboard too low will add drag and reduce performance.

Prop selection for a dinghy is usually a challenge and often a compromise. In some cases, the combo that gives the best performance heavily loaded will cost you some speed when running solo (either due to a more draggy 4 blade prop, or due to needing to run less than WOT when solo to stay off the rev limiter).

[markcouz]

Thanks, I already have the Doelfins installed but will take your reply as a vote for the 3 x 8.5 x 6 - I just dont want to find that I have gone too low in pitch so still hoping that someone with similar issues can let me know their solution.

[markcouz]

Originally Posted by rslifkin

The only way to really get it right is to buy one of the small inductive tachometers and install one on the engine. Then you'll know what RPM you're seeing at full throttle with various loads.

If you're just not making enough RPM, you probably want a lower pitch prop (or add trim tabs to the dinghy for more lift to help it plane at a lower speed, or both). If your RPM is good but it's still struggling along and can't get out of the hole then it might be worth going to a 4 blade to get more bite (the 7 pitch 4 blade would likely be a good pick in this situation). Solas makes props for that outboard that give a few pitch options, etc. that aren't available in the OE props (and at least on the 6hp, some people have found the Solas props to turn more RPM and run faster than the same pitch/diameter OE Tohatsu prop).

Play with your trim position on the outboard as well as the mounting height (ideally you want it mounted high enough that the anti-vent plate isn't too far below the water surface while on plane lightly loaded). Having the outboard too low will add drag and reduce performance.

Prop selection for a dinghy is usually a challenge and often a compromise. In some cases, the combo that gives the best performance heavily loaded will cost you some speed when running solo (either due to a more draggy 4 blade prop, or due to needing to run less than WOT when solo to stay off the rev limiter).

Thanks for your detailed response - more for me to investigate than a quick fix I quess!

[Jammer]

Long shaft motor will perform poorly if your dinghy, like most, has a low transom and is designed for short shaft. If that is your situation then you should sell your motor and buy a short shaft one, or convert yours to short shaft if you can get the parts.



If you want to experiment with props, get a combination tach/hour meter like the one in the photo. They cost around $20, attach with velcro, and have a wire that you wrap around one of the spark plug wires on your motor.


Then take your boat out and see what RPMs you hit when you're in the situation loading wise where you want to improve performance, and see what RPMs you hit while alone. Compare these to the target range in the specifications for your engine.


Reducing prop pitch by an inch will typically increase your RPMs by around 800-1000. If your RPMs are too low you'll pick up one or two HP.


If you can reduce pitch and still avoid overspeeding the engine at WOT with one person, then it will be an overall improvement, and is worth doing.


If reducing pitch will end up overspeeding the engine at WOT with one person then at best you'll sacrifice some performance with one person and pick up some performance with more people. Usually it isn't worth it, but it's cheap to experiment and then put the old prop back on.


A fact to consider is that prop performance falls off quickly with even relatively minor damage to the blades. If your prop is chewed up from close encounters with rocks, you should replace it with a new one that's the same pitch before you start experimenting.

[rslifkin]

Originally Posted by Jammer

Long shaft motor will perform poorly if your dinghy, like most, has a low transom and is designed for short shaft. If that is your situation then you should sell your motor and buy a short shaft one, or convert yours to short shaft if you can get the parts.

Good point, I didn't catch that this is a long shaft. If it's a long shaft mounted on a dinghy meant for a short shaft, yes, it will always perform poorly. There's far too much lower unit in the water providing drag (and with it that deeply submerged, I wouldn't be surprised if the fin makes things worse, not better). And the thrust is being supplied far lower than expected as well.

[Cheechako]

With that 10.5 ft dingy you need a 15 hp, short shaft. Been there, done that with the 10 hp. The shaft length even makes it worse.. Mine was an old school 10 ft + AB double floor RIB. I think it weighed 136# empty.
Also:
I had a couple extra props for my 15HP. One eventually got pretty beat up, so I put a spare on. The spare I bought on line cheap although brand new was another Yamaha prop. It was way different specs than the one I took off. Running the boat you wouldn't know it was.

Just sayin', Maybe you can get there, but maybe not. You could extend your transom up and try the long shaft that way I suppose.

[IslandHopper]

Originally Posted by markcouz

I have a 3.2m dingy (glass v-hull) with Tohatsu MFS9.8B 4 stroke long shaft engine

Originally Posted by Jammer

Long shaft motor will perform poorly if your dinghy, like most, has a low transom and is designed for short shaft. If that is your situation then you should sell your motor and buy a short shaft one, or convert yours to short shaft if you can get the parts.

Originally Posted by rslifkin

Good point, I didn't catch that this is a long shaft. If it's a long shaft mounted on a dinghy meant for a short shaft, yes, it will always perform poorly. There's far too much lower unit in the water providing drag (and with it that deeply submerged, I wouldn't be surprised if the fin makes things worse, not better). And the thrust is being supplied far lower than expected as well.

This was the first thing i noticed and coincidently the same issue another recently had at our Marina, he'd bought a second-hand LS outboard for his RIB without realising it and the thing performed like a pig.

No amount of him experimenting with props and foils could fix the issue until a mate of his pointed out he had a Long Shaft OB instead of the correct Short Shaft, when they borrowed my Short Shaft OB of the same HP it performed as it should.

In my opinion until you get it checked out for the correct length outboard i would not waste anymore time and $ experimenting with anything else.

[markcouz]

Thanks for all the suggestions.

The dingy was manufactured on my order, and the transom was raised to accommodate my existing long shaft engine, so I hope that it is not a shaft length issue.

I am going to try and get the rpm gizmo (could be tricky here in Grenada) and start investigating from there using the info that I have learned here.

[Scubaseas]

Your underside of the cavitation plate ideally would be about 5mm below the bottom of the boat. I agree with most all the advice here. Lower pitch might do it but if the motor is sitting too low no prop is going to fix it.

[markcouz]

Originally Posted by Scubaseas

Your underside of the cavitation plate ideally would be about 5mm below the bottom of the boat. I agree with most all the advice here. Lower pitch might do it but if the motor is sitting too low no prop is going to fix it.

Thanks, I will check that spec this morning.

[rslifkin]

Mounting height is a bit of a black art, as every hull, engine, and prop combo will like something slightly different. If the anti-vent plate is anywhere from a bit above to a bit below the bottom of the dinghy (no more than an inch or so either way), you're at least in the ballpark for a small outboard. You can try raising the outboard up with a 1/2" spacer on top of the transom and see if you gain any speed. Just make sure you it doesn't lose cooling water at high speeds and that the prop doesn't ventilate too easily in a turn. If either one of those happens, the outboard is mounted too high.

Raising the outboard as high as I could still safely mount it (got the anti-vent plate just level with the bottom) on my dinghy was good for an extra 1/2 knot when lightly loaded (didn't do a comparison when loaded more heavily). It also changed the fastest trim setting from being in hole 4 to hole 3.

[markcouz]

I have just measured transom height and it is the standard 20" required for long shaft engine - I will experiment with 1/4" and 1/2" spacer before moving on to [3 x 8.5 x 6.5] or [4 x 8.5 x 7] depending on RPM results - and finally 15HP if all else fails.

[Jim Cate]

FWIW, some years ago we had a Suzuki 15 on a big heavy Mk2 Zodiac. With the standard prop we couldn't get on a plane with 4 adults aboard. Bought a 4 blade prop with cupped blades from Solas, using their advice about pitch and diameter. To our surprise we could then easily plane with all aboard, top speed was better and we were easily able to avoid overrevving when lightly loaded. You might have a chat with them...

Jim