Dinghy Propeller Pitch

[Phase3]

I have a Yamaha F20 on a 10.5 Highfield dinghy. All has been good over the last 8 years with just my wife and dog and I in the dinghy, however with 3 adults, the boat will not get up on plane and motor “bogs “ down.
The prop is a standard 9.25 x 10 three blade.
What pitch of prop would you suggest to get up on plane?
Is a four blade prop better than a three blade?
Motor has a fin already.

Brother has same size Walker bay with Honda 20 and four blade prop of a lesser pitch and he can easily get up on plane with 3 adults.
Thanks in advance everyone

Brent

 

[rslifkin]

The first step should be to get and install one of those little battery powered induction tachs. Then you'll know what RPM it's reaching at WOT with various loads. That will guide you to a correct prop.

Our 6hp Tohatsu is running a 7.8x7 prop (1 inch less pitch than the original prop). With just 1 person, it's underpropped and will hit the rev limiter if you don't back off the throttle a little. That gives up about 1 kt at WOT compared to the 8" pitch prop. But with a heavier load the 7" pitch is significantly better. It's faster with 2 adults + dog, plus the maximum weight to plane is higher than the 8" pitch. And the 7" pitch still gets solidly into the recommended WOT range with 2 adults + dog, while the 8" pitch barely did (and didn't quite rev to the listed max HP RPM with that much load).

 

[fgarriso]

Full Throttle RPM Range..............................5000 – 6000.
Try to get close to 5500 RPM.

 

[tiltrider1]

Reduced pitch is the answer. The four blade vs 3 blade isn’t going to make the difference.

 

[Grupet]

I have a 10.5' Alliance. It is a deep vee fg rib with a 20 Tohatsu. I added the tach as mentioned above to determine I was near the top of the rpm range at wide open throttle, running lightly loaded.

I would ad that not all props are equal. On my boat, the oem 9.25x11 worked well. However, after a few visits to the sandbar, i needed a new prop. I tried the Solas brand Amita prop. The same size (9.25x11), the engine hit the rev limiter. I exchanged it for a 12 pitch, which works great on my boat. Visually, the blade design of the Solas is different than the Tohatsu oem prop.

An AI search indicates the Tohatsu has a 2.15:1 gear ratio while the Yamaha has a 2.0:1 ratio, so I would expect the ideal props to be different, even if the boats and horsepower are similar.

I found that all prop designs do not deliver the same results, even with the same dimensions. If I were in your shoes, after confirming your rpm running light, I'd be inclined to visually compare the blade shape of the yamaha to the Solas prop and order accordingly.

Hope thats useful.

 

[jgwinks]

Very generally speaking, reducing pitch will allow the engine to run higher up in it's power band and produce more horsepower. They sometimes call this a "power prop". This may however reduce your top speed somewhat. As mentioned above you need to know what rpm you're reaching under various load conditions and try to get the engine up to 80-90% of is max speed. You'll probably find that with the original prop you're not getting that even when lightly loaded. Going to a 4 blade of the same diameter and pitch won't help, and could even make it worse.

 

[Dougcole]

Have you tried a doelfin or similar? I have a 20hp mercury on our Highfield 340 cl and it gets easily up on plane with three adults and a load of dive gear. I added the fin and it helps a lot, I can get it up on plane with 4 adults now, albeit barely.

The tach is a good idea too, and a new prop might help as well, but I'd start with the fin. Easy install and pretty inexpensive.

 

[High Wire]

+1 on those cheap induction tach/hourmeters from Amazon. Every dinghy should have one. Just make sure you get the initial setup correct.

 

[Dougcole]

Sorry for thread creep, but have any of you ever extended the signal wire for one of the little induction tachs? I bought one and would like to mount it on the little storage box I use for a "console." I need to add about 4 feet.

The wire looks to me like a single stranded wire, about 18 gauge, can I just use a butt connector? Is there a voltage drop issue?

Thanks.

Doug

 

[MOJO]

Reduced pitch is the answer. The four blade vs 3 blade isn’t going to make the difference.

This is the answer. I had the same problem with a Yamaha. Went to a reduced pitch prop and it now runs like a spotted ape! Forget about 4-blade, that just reduces efficiency. The purpose of adding more blades is to smooth out vibration, not increase power.

 

[rslifkin]

The purpose of adding more blades is to smooth out vibration, not increase power.

Or to get more blade area when going to a larger diameter prop isn't an option.