Prop Pitch on Dinghy

[By Invitation]

I was hoping someone could help me out here. I have an 18hp 2 stroke Nissan with lifters on the lower end (I love these) on a 10' hard bottom dinghy. I want to change the pitch to give me more power and less speed. The dinghy right now screems along with 2 adult people, it almost goes too fast to be safe. With 3 adults it gets up on a plane and runs well. With 4 adults we cannot get up on a plane. I would like to reduce my prop pitch (currently at 9.0) to enable me to get up on a plane with 4 adults. Does anyone have any good idea of what pitch I should go down to? Trial and error can be expensive.

Thanks

[Alan Wheeler]

It is really hard to estimate with a dinghy. A prop shop should be able to get you close. You take your old prop in and ask them to make an estimate. If you are not too far from them, then really it is a case of trial and era. Hopefully they will allow you to try and try and try.
However, in my guess, reducing by just 1" will make a dramatic improvement.

[By Invitation]

Thanks Wheels,

My guess was an inch also. I don't have any prop shops nearyby so I am hoping that someone on this board has done this before and can advise me from first hand expeirience. The props are not cheap so I need to avoid buying more than one.

[David M]

A lot of prop places and outboard repair places have props that they will allow you to borrow for just his purpose...so it does not become hit or miss with your checkbook.

[daydreamer]

We have an 12' fiberglass AB RIB with a 15 HP Yamaha 2-stroke outboard. We spun the factory prop (aluminum, 9x11) and replaced it with an aftermarket Piranha prop (composite, 10x10.5). We noticed a significant drop in top speed. And it is just a little more difficult to get up on a plane with the dinghy loaded (4 heavy or 5 light people).

So, first I'm going to investigate having the hub replaced in original prop. Then I'm going to look into swapping out out the blades on the Piranha prop for a different size/pitch combination; unfortunately, they do not make a 9x11.

[marshmat]

4 adults @ 180 lb each, plus ~100 lb motor, plus ~50 lb gas and gear, plus ~130 lb boat at 10 ft LOA. Say 1000 lb total. With 18 hp, you're at 56 lb/hp.

With those numbers, it should be able to stay on plane once up, but it'll have a hard time getting there.

If I'm looking at the right charts, the Nissan 10-20 hp models are designed for a 9.2" diameter prop. A very rough calculation would suggest a 9" diameter 10.5" pitch 3-blade for top speed (approx 19 kt at 1000 lb all-up). If your current one is 9x9 and can't get her up on plane, I might try dropping pitch to 8" and recognize that you'll have to shift some weight around to get her up at full load.

[djmarchand]

I bought a flatter pitch prop from Nissan for my Nissan 9.9 hp two stroke. It made a dramatic difference in planing.

I suspect that Nissan has an off the shelf flat pitch prop for your engine.

David