Drago Fiesta and Yamaha O/B, part two

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clb1970

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Ok, I posted before about a seeming lack of 'go' with my yamaha F50 o/b on a Drago Fiesta 600 (20 foot GRP day cruiser)

More testing and trialling showed her doing a max of 12 knots and a maximum RPM of 4300. She'll happily rev to 6000 RPM with no load.

As advised I've fitted Doel Fins with some effect, the stern is getting higher now, but we're still not properly planing.

The prop is an 11 5/8 x 11 which is the lowest pitched prop Yamaha do for this fitment.

We've taken the hood off, checked and cleaned the fuel filter (tiny bit of rubbish there, certainly not blocked) The engine is getting full throttle.

So, what's next?
Prop change I think, but to what? I could go for a 12 x 8 prop, but would that just give me the same or similar speed but with max revs, or do I go for maybe an 11 x 13 or 11 x 15 prop hoping that it'll give some extra speed on the way up, get me planing, reduce drag and therefore give a better end result?

Confused? I know I am

Thanks!
Chris
 
I'm afraid you have done pretty much all you can. This engine/ boat combination is unlikely to get fully on the plane, the engine is just not powerfull enough.

I would think you will need at least 80hp and more comfortably 100hp to get the boat fully over the hump and on the plane.

Sorry if its not what you want to hear, but spending another £100 on a different prop may only gain you another 2-3 mph.
 
Someone has suggested that it could be an ECU problem. Is that likely as it runs beautifully, just won't pull to max revs under load. I'd have thought that it maxes under no load would suggest that things are ok internally.
 
i once had a problem with a shetland, like yours and i got 2 stainless fixed trim tabs made, which could be manually adjusted via bottle srews, this worked for me a lot better than doel finns. better option than keep changing propellars, have you spoken to yamaha, they can give the best prop sizes.
 
That's the next step. The Yamaha guy at my marina is back this week. So hopefully he'll have some ideas.

I know she's never going to fly, it's just maximising what we have
 
Unfortunately I have to agree with other replies, you simply have not got enough horses for a 20ft boat, its tough but you have to face facts. You need in the region of 100 hp to acheive 20 knots, I know, I have been there and done it! With my 21ft Cranchi and 140hp we cannot get above 26 knots, it is simply underpowered. Although I am sure you should be getting more revs at WOT.
 
Yeah, reconciled to that, it's now about maximising what I have got. No point losing 1000 RPM at WOT if I don't have to.
But thanks for the conformation that it's not just me

Chris
 
If max revs with your engine is 6000 and that is where it develops max power and you are only getting 4300 you aren't seeing max power. I suggest your prop is too coarse a pitch. See if you can borrow a finer pitch prop so your engine approaches the 6000 rpm mark if that is what it is supposed to do. The 12 x 8 might do it, you'll have to try it out. Once under load your engine develops max revs that's all you can do there.

I go along with the other poster in that rigging screw controlled fixed tabs could be a cheap and easy way to get the stern up so she rides level, I did mine years ago in aluminium on a 20ft boat. What I did was use a thin piece as a "hinge" with a thicker piece over to stiffen it bolted through over mastic into the transom, and a 6mm thick piece riveted/bolted to the hinge with another thicker piece the other side (on the top). Some pieces of "T" section riveted to the fin provided a place to mount the clevis and yoke of the rigging screws, with small pieces bolted to the transom to carry the other end. I started with the rigging screws at around 45deg and fins down about 5 deg and experimented from there. Fin size was about 200mm x 450mm wide with two rigging screws on each. The "hinge" just bent to allow adjustment. Worked fine for me. Stainless is no doubt better if you are permanently afloat but if trailed aluminium is fine and easier to work with.
 
I agree with Cliveshep.
You are only achieving 4300 rpm.
The prop pitch is too coarse.

Finer pitch will give you more revs.
1 up with a 50 on your boat?

Blimey it should plane.

imho.
 
As the others have said you have got to try a smaller prop - a bigger pitch prop will makes things worse.

I´m suprised you say that 11 5/8 x 11 is the smallest Yamaha do.

There is more info here on the Yamaha website. It looks like you can get 8 and 9 pitch propellors, which may give you the extra torque you need to get planing.

There is also no point in messing about with ECUs and other bits and pieces on the engine until you have tried a smaller prop. Anyway, a spare prop is always handy to have.

Just to add, ebay or boatsandoutboards are both good sources for second hand props.
 
In theory without prop slip Usually around 10%. Assuming your gear ratio is 1.85 to 1.
An 11 inch pitch prop would give you 33.78 mph
An 8 inch " " 24.57 mph
if the engine is pulling 6000rpm.

Just a thought, is the cavitation plate the correct height and not too deep?
 
[/quote]As the others have said you have got to try a smaller prop - a bigger pitch prop will makes things worse.

I´m suprised you say that 11 5/8 x 11 is the smallest Yamaha do.

[/quote]

Ah, no. What I meant was the 11 pitch is the smallest in the 11 5/8" diameter. To go finer than that I would need to go to a 12" diameter, which I'm not sure what the effect would be. Am I right in thinking a 12" diameter would also effectively increase the revs?
 
The plate seems at the right height. Unfortunately I'm inheriting all the previous owners set-up.
I think I'll go talk to Mr Yamaha tomorrow and see if he has a couple of props he can let me try.

Good point on the spare prop though, Sounds like a very good plan!
 
Am I right in thinking a 12" diameter would also effectively increase the revs?
Click to expand...

Going from the 11 5/8th to 12" diameter would only increase the diameter by 3/8th of an inch. I doubt if you would notice the difference, but if you did it would tend to reduce the revs as you have more drag from the prop. The big change is dropping the pitch from 12" to 9" or 8" which in percentage terms is much bigger than the change in diameter. And if you can borrow a prop to try this out, so much the better :)
 
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