More top end?

[Windsurfer]

Hi All
A couple of summers ago i replaced my 3.2 honwave powered with a 15hp 2 stroke with a new 3.8 quicksilver alu hull rib and a 25hp yam 2006 2 stroke.
The rib has been great all round but felt it lacked a little top end , roughly the same as the 3.2 honwave set up at 20knts.
Quick search on google and brief chat with other boaty types seems to think i should be getting around 25knts with the 25hp yam on the quicksilver.

There was 2 people on the boat when testing the quicksilver (both 70kgs) in wetsuits, 10 litres of fuel and nothing else.
We switched trim positions for best results at 21knts.

The current prop is 9 7/8 × 14 - F and in the manual it says i can go another pitch up to a 15, would that make much difference?
Theres plenty of low down power at the moment so not fussed about losing a little.

Also my av plate sits 18mm below the hull, there's enough space on the transom plate to pack the outboard up around 7-10mm but not really keen on doing this as i like it all sitting down tight on the transom.

Any thoughts would be much appreciated.

[Fenlander]

I never like to fiddle with a prop size/setup unless I have a tiny tach (well a budget version) to know where the revs are.

[Limecc]

As Fenlander says you really need a tacho to determine where you're at. If it could pull through to max 5500 rpm then the 14" prop is good for almost 26kn at 15% slip. Google is correct.

If you're only getting 21kn then it's just shy of 4500rpm and a 15" prop will make it worse, at those revs it might only be making 18hp. A 13" prop would give you at least 21kn, max 24kn and is the way to go.

Why not try a Hustler? Yet to try mine but great reviews and a bargain price from Streamlined Propellers. Check them out!

[Scooby]

I have same boat and my 15 hp Yamaha gets 22 mph at wot 5250 rpm
So very interesting because I've just got a 25 hp Yamaha to go on my boat

[Scooby]

On last year's ribnet trip I had about 290kg with fuel camping gear and my 15 hp Yamaha 2 stroke managed 19 mph at wot would have expected 25 hp to be much better

[Limecc]

Originally Posted by Scooby

I have same boat and my 15 hp Yamaha gets 22 mph at wot 5250 rpm
So very interesting because I've just got a 25 hp Yamaha to go on my boat

22mph = 19kn. You must be running a 11" prop. Good choice.

[Scooby]

Have to say a bit worried now was hoping the 25 hp Yamaha 2 stroke I've just got not even on the boat yet would provide me with more power what's the standard prop for this engine as propis not stamped with size

[Limecc]

Originally Posted by Scooby

Have to say a bit worried now was hoping the 25 hp Yamaha 2 stroke I've just got not even on the boat yet would provide me with more power what's the standard prop for this engine as propis not stamped with size

Why are you worried? It is what it is regardless and if you've got the right prop at the max rating for the transom and are still disappointed then you have to either swop boats and get a bigger engine or set different expectations. You're not confusing knots and mph are you?

There is no 'standard' but I expect a middle-of-the-road prop for the Yam 25 would be 13" and it should pull a 14" if on an efficient hull and lightly loaded..

I recall my 30hp RIB used to give me 26kn, repowered with a 40 I got 28kn, my current 50hp boat runs to 30kn.

[Windsurfer]

Thanks for the reply’s , my current prop is a 9 7/8X13 pitch (not 14 as I put above) , I was given a spare 11 pitch with the outboard but I hadn’t tested it as I was under the impression that less pitch would give me less top speed?

Is it worth testing the 11 just to see the results?

[Windsurfer]

Originally Posted by Scooby

Have to say a bit worried now was hoping the 25 hp Yamaha 2 stroke I've just got not even on the boat yet would provide me with more power what's the standard prop for this engine as propis not stamped with size

I have a pdf work shop manual if you want me to email it to you?

[Scooby]

What model is your 25 Yamaha please as I've just got one to go on my boat

[Windsurfer]

Originally Posted by Scooby

What model is your 25 Yamaha please as I've just got one to go on my boat

It’s a 2006 25hp BMH , have you also got the 3.8 quicksilver ?

[Windsurfer]

Originally Posted by Fenlander

I never like to fiddle with a prop size/setup unless I have a tiny tach (well a budget version) to know where the revs are.

Thanks Fen , just had a quick look. We’re is the best place to mount the display ?

[Scooby]

Hi yes 3.8 ALU rib and same Yamaha mine is 2007 with 11 hours on it

[Windsurfer]

Originally Posted by Limecc

As Fenlander says you really need a tacho to determine where you're at. If it could pull through to max 5500 rpm then the 14" prop is good for almost 26kn at 15% slip. Google is correct.

If you're only getting 21kn then it's just shy of 4500rpm and a 15" prop will make it worse, at those revs it might only be making 18hp. A 13" prop would give you at least 21kn, max 24kn and is the way to go.

Why not try a Hustler? Yet to try mine but great reviews and a bargain price from Streamlined Propellers. Check them out!

Sounds like your bang on with with the speed / prop size , I had a 13 on and not 14 and getting 21knts. What are you thoughts on trying my 11 pitch ?
Going to order a tacho so I can get some actual figures etc.

Will also take a look at the hustler , keen to get my set up running as best / efficient as possible

[Windsurfer]

Shame your not local to me as we could of done some side by side testing etc , but would be good to share our findings along the way !

[Scooby]

Would be great to share hopefully be testing next weekend
As have ribnet trip on 2 April no rush then

[beamishken]

You need to move the engine up the transom to get the best out of it, a bit like running with the handbrake on running too deep.
You might not want to but its your cheapest option to improve

Sent from my SM-G950F using RIB Net mobile app

[Limecc]

Here's what you can expect from different props with estimated 15% slip. Yam 15 and 25 share the same 2.08 gearbox ratio and the WoT range is also 4500-5500rpm:

Pitch - WoT range (kn)
10" = 15.1 - 18.5
11" = 16.6 - 20.3
12" = 18.2 - 22.2
13" = 19.7 - 24.0
14" = 21.2 - 25.9
15" = 22.7 - 27.7

If you are now saying it's a 13" prop, then you're just into the WoT range, perhaps by a little, perhaps by a lot depending on what your slip is and we just don't know without a tacho.

If you need to bring excessive slip down a larger diameter smaller pitch is what's needed or perhaps a four blade and smaller pitch. The 11" prop will never give you more than 20kn so that's not going to help.

Originally Posted by Windsurfer

Also my av plate sits 18mm below the hull, there's enough space on the transom plate to pack the outboard up around 7-10mm but not really keen on doing this as i like it all sitting down tight on the transom.

Absolutely. But raising 3/8" (1/2 of one hole) not going to help enough. Needs a lot more. Maybe best to drill the transom rather than using clamps. See transom pics of my last two boats. Not possible to use clamps and get the right height even if they were fitted.

[Limecc]

Here's a transom drilling template. My holes are drilled black and yellow which allows me to run my AV plate +40mm above the hull in the highest setting, two holes down is level and it's -20mm below on the lowest.

I think you will be safe starting 20mm higher than level and if you are lucky it still won't ventilate in the highest position.