Prop size help.

[36valley]

Prop size suggestions needed from you guys if possible.

Long story short, have a 2019 Suzuki df150 with a 15 1/4 x19 stainless prop on it. Engine bounces off the rev limiter at 38.5 knots at less than full throttle. Dealer thinks this is perfectly acceptable! Is it?

Anyway I'm resigned to sorting a more suitable prop myself. Top speed isn't that important, economical cruising in the mid twentys is what I'm really after. That and not over revving it when I do open the throttle with a little enthusiasm!!

The rib is 6.5 metres, two jockey seats but mostly just me onboard, 160 litre tank but carry an extra 100 Litres in Jerry cans for longer trips, which it was built for.

Any thoughts and suggestions gratefully recieved.

[Roflhat]

Could go up to a 21, will give better top end if you're hitting the limiter and lower your cruising revs by a few hundred rpm. Of course higher pitch means slower acceleration but if you've got light load it's not so much of an issue

[36valley]

Originally Posted by Roflhat

Could go up to a 21, will give better top end if you're hitting the limiter and lower your cruising revs by a few hundred rpm. Of course higher pitch means slower acceleration but if you've got light load it's not so much of an issue

Thanks for the reply.


21 was my thought too. My concern is it might not be enough as I'm only able to use 3/4 throttle at the moment. The acceleration and getting on to the plane isn't likely to be an issue as scalded cat comes to mind when you open the throttle at the moment.

I did wonder if maybe a 4 bladed prop might help with mid range economy or are they more suited to heavily loaded boats.

[Roflhat]

You'll generally get worse economy with a 4 blade prop, better acceleration and hole shot, for pulling skiers etc.
Could try 23p, it's a big jump but if the 19p is really too small might be the way to go

[36valley]

Originally Posted by Roflhat

You'll generally get worse economy with a 4 blade prop, better acceleration and hole shot, for pulling skiers etc.
Could try 23p, it's a big jump but if the 19p is really too small might be the way to go

That will teach me to listen to Americans talking about prop sizing on youtube.

I am wondering if going to a 23 in alloy instead of stainless might be a less expensive way to experiment. I believe that their less efficient than stainless which might make the best compromise.

Any suggestions on a reliable source for props online?

[pagick]

I always look at eBay for used. For a little hassle you can try and flip sizes that don’t work back on eBay, no loss except shipping.

[36valley]

As these sort of threads often don't seem to have a conclusion and just in case anyone might have the same query in the future I thought I'd add the result of my question.

I managed to get a good deal on a new 21" Suzuki Watergrip prop. So was replacing like with like, just the 2" increase.

I fitted it yesterday and put 100 miles on it today.

The result was exactly what I was looking for, maximum revs are now 5900, so no more hitting the rev limiter. Top speed with 3/4 fuel and one big bloke onboard has gone up to 41 knots. A lighter load should give a knot or two more I think. The previous top speed was with less fuel on board.

Acceleration is still rapid, so no problems there. Fuel consumption has improved by roughly 10%. Not massive, but every little bit helps. Cruising in the low to mid twenties is as predicted, at a few hundred revs lower, obviously helping the lower fuel consumption. At 5 knots the fuel saving seems to be even greater.

What did come as a pleasant surprise was it now stays on the plane better at lower speeds. Before if I was cruising at 16 or 17 knots, it wouldn't take much of a wave to knock the speed off and to find myself dropping back to 12 to 14 knots and needing to add some revs to get back up to speed. That seems to not be happening so often now.

All in all, I'm really pleased with the result.