Choosing the correct prop pitch

[daviesd]

Hi guys

I'm hoping for some advice on this one.....

I run a Tornado 6.5 and have recently put a Yamaha VMAX 150 TRP ( counter rotating props ) on the back. The engine seems to run great, but the problem I have is that it won't rev past 4200rpm. There appears to be nothing wrong with the engine, it revs past 6000 in idle, so I can only assume it is the prop size. It is fitted with a pair of 25 pitch props!!!

I am a little confused as to why it is not performing as it has come off a Ribeye 7.25RS rib where it was revving to about 5500rpm ( about 46 knots ).

Due to the 'special' nature of this engine, the are only 4 pitch sizes available, 19,21,25 * 27.

Anyone one have any ideas???????

thanks

David

[Pete7]

David, no sorry other than that it could get very expensive very quickly

Just one thing worth checking though before you start with props, is the fuel line a decent size throughout its length, like 3/8 bore, it might not be getting enough fuel for high speed but just enough to allow the engine to run at WOT in neutral for a few seconds.

Easiest way might be use a quicksilver bayonet 25 litre tank straight to the engine for a quick run.

And lastly does the control box hand throttle move the accelerator as far as the normal throttle or further ?

Didn't Manos have one of these engines ?

Pete

[daviesd]

Thanks Pete

I emailed Manos ( as a newcommer, how come he is suspended - politics I guess ) and he is having no problems with the engine on a falcon race rib - 63kts at 6100rpm.

I think the fuel is ok. I fitted a navman fuel meter and can see that when the throttles are open full ( it does appear to do full travel ), the consumption jumps to 63 litres / hour for little or no extra speed so I guess the engine is under stress trying to spin the props.

The other option is to raise the engine as it does have a very low intake and I think these things are designed to run high.

WHat impact on handling would you expect to see as the engine height is raised ( not trimming ).

thanks again

[Pete7]

If its raised then the props will ventilate more and loose grip but, you have twin counter rotating props which might alow you to get away with it. Got to be worth a try, its only ten minutes work. Put the engine leg on the deck and use the trailer jockey wheel to lower and raise the boat. You need two friends to help.

Pete

[jwalker]

Can't speak for counter rotating props but for a single prop on a 150 25" sounds too much unless the boat is very light. If you can glean any relationship between the single/counter setups, let us all know for info.

[daviesd]

thats sort of what I am thinking - I don't think the Tornado's are particularly light. Although, it is kind of bizarre that it ran fine on the larger Ribeye.

I did notice that around the stern of the ribeye, it looks like it has a VERY pronounced step which might reduce its planing area - maybe???

[jwalker]

Yes, that's a puzzle. I have no experience of the Ribeye range. Are you sure it was running the 25's when on the Ribeye?

[Bilge Rat]

Have to agree with JW a 25" pitch seems very high, if you drop the pitch down the engine will rev higher but you will loose top end speed, raise the pitch and you loose 400 rpm but gain top end speed..... supposedly that is.

of course as someone already said this is going to get very expensive for you, best bet in my opinion is to try everything else suggested before going and buying new props, then if the problems still the same call Julie at Steel Developments and present the problem to her, she'll be able advise the best course of action, one thing though you really need to be clear on what your trying to achieve, i.e top end performance or excelleration etc.

Twins...... Ouch thats gona cost

[thorper]

David

The hull on the tornado will be "slower" than the ribeye sport - the tornado is designed for load carrying rather than running fast, especially if the ribeye has a stepped hull. (The falcon hulls manos talks about are also meant to be pretty quick)

We've got a 19" prop on our yamaha, which gives 53mph at 5500rpm, although its not counter-rotating, so probably not so easily compared. If you search for some of the discussion threads on props on the forum (several in the past) you'll find some good info (from memory 1 inch of pitch up or down gives/takes away 100-200 rpm)

Ricky

ps are you the David Davies that worked (still works at?) RBSI in 2001?

[daviesd]

Hi Thorper - yep, still working at RBSI. gotta make some cash to run this thirsty engine. Where do I know you from then?

I think I can get a set of the 21" props from the US for about 600 big ones ( including shipping ).

Not really that fussed about the top speed as its mainly for diving / freediving etc. but I just want the engine to be running properly, ie not over stressed.

I checked with the previous owner and they are definately the same props.

THey do make for relaxed cruising though - 3200 rpm and just under 30 knots - keeps the fuel useage down I suppose

[jwalker]

Yep, 19" is what I'd expect too.

Rat is not quite correct but I know where he is coming from. Reducing the pitch, at the same rpm, will cause the boat to go slower but you will benefit from higher revs. When correct, the benfit from higher RPM, and therefore power, will outweigh the loss from pitch.

Fuel economy will be ok because the engine will be working lightly on part throttle when cruising. You might find it sounds less grumpy too.

For diving, ie. load carrying, a bit low on pitch rather that high would be better.

[Bilge Rat]

Yep i was kinda right, this time last week i knew nothing about pitch or diametre, been trying to find a resonably priced prop for my engine, currently have an ali 11-1/2x19 and wanted to go stainless to a 21 inch pitch, the boats light and runs fast as it is just wanted to see if i could increase the performance and technically i could, problem is though that i cant get a 21 or 19 in stainless with a 11-1/2 cm diameter unless of course i want to have it shipped from Belgium or the states at that obviously increases the price.

[daviesd]

Yeah, SS props do seem real expensive. As far as I can tell, I can't find any props for this engine in the UK. They are 14 * 25 and 13 * 25.

The postage from the US isn't too bad if you use the US Postal Service and not carries like Fed-Ex or UPS. I think to ship both props ( about 35lbs ) it will cost $100 for 6-10 day delivery. I guess a single prop would be around $60.

[thorper]

Yep I agree with JWalker - a lower pitch will give you more acceleration, therefore get you up on the plane quicker when the boat is loaded.

We get pretty good fuel economy revving about 3500, giving 30knots (ish).

David - I was working on the ePay team for 4-5 months back in 2001.

Ricky

[daviesd]

Rich - I thought it might be you...... Are you still with IBM???

You doing much diving?

[thorper]

Dave - sent you a forum private message thing

[tim griffin]

i currently have a 21 on my 6.9 and am changing down to a 19 as we Solent and me want to have a play and decide what will suit her better for the work she does , i am leaning to the 19 more and more at the moment we will be doing this at ribx so come along and see what we come up with.
regards tim
www.griffmarineservices.co.uk

[jwalker]

Originally Posted by daviesd

Yeah, SS props do seem real expensive. As far as I can tell, I can't find any props for this engine in the UK. They are 14 * 25 and 13 * 25.

Are you saying that different pitches are not available but different diameters are?

The postage from the US isn't too bad if you use the US Postal Service and not carries like Fed-Ex or UPS. I think to ship both props ( about 35lbs ) it will cost $100 for 6-10 day delivery. I guess a single prop would be around $60.

My torqueshift prop came over from US at about £30 by UPS, albeit a while back.

[DJL]

Originally Posted by daviesd

thats sort of what I am thinking - I don't think the Tornado's are particularly light. Although, it is kind of bizarre that it ran fine on the larger Ribeye.

I did notice that around the stern of the ribeye, it looks like it has a VERY pronounced step which might reduce its planing area - maybe???

The 6.5mtr Tornado is about half the weight of the 7.25mtr Ribeye.

[jwalker]

Originally Posted by DJL

The 6.5mtr Tornado is about half the weight of the 7.25mtr Ribeye.

Don't tell him that. Now he'll be really peed off.