Who else is running a single Yamaha F250?

[Richard Selman]

What prop are you running and what performance are you achieving?

The 2:1 gearbox is turning the prop a bit slower than other similar engines and its making it difficult to get the prop selection right based on what others in this forum. I am current running a 23" P * 14.2D alloy prop only achieving 38 knots at the 5500 rev limit i.e. slip = 26%. Osprey think I should achieve 47 knots with this engine. Roy sent up a prop to try but the shaft fitting does not match. Noone else I know round here runs an engine anything like this size so can't borrow a prop to try.

Want to change to a stainless prop but I would like so see what others are using before I blow another 300 quid.

I think I need to increase the diameter and not sure how far. Most of the prop discussions concentrate on pitch and largely ignore diameter. I would have thought that diameter or blade area was as critical to getting the right performance as it would set the ability to deliver to power.

Cheers
Richard

[Roy Smith]

We had the same problem with props, Scorpion tested a lot of props with our rib
but speed at WOT differed a lot
Standard three blade black S/S prop 21" WOT 39 knts
Scorpion had to drop the engine to get more grip

we now have the same prop as you which i think is about as big as you can go to
23" 14 1/4 Yamaha made S/S Prop but top speed depend a lot on how much fuel
you carry and how many people you have on board.
we have had 47/48 Knts pulling 5800 rpm 1/4 tank of fuel two up flat sea
but full tank and four people 42/43 Knts WOT 5600 rpm
I know that steel developments have a test list of most boat engine
and they would be able to advise you
I don't about Vipermax but i think Yamaha are very sensitive to set up

[Doug Stormforce]

Originally Posted by Richard Selman

What prop are you running and what performance are you achieving?

Want to change to a stainless prop but I would like so see what others are using before I blow another 300 quid.

Richard pm me your email address and I will send you an attachment with some acceleration curves for that engine with various props.

[Richard Selman]

pm sent.

[codprawn]

This is what worries me about some engines. Using the prop calc programs it all depends on gear ratio. It seems to me you will never get much speed out of a low ratio becuase the prop just isn't spinning fast enough - a bit like trying to get a car to go fast in 1st gear.

When I was comparing the Suzuki 4 stroke to a 300 2 stroke the gear ratios were very different.

[Richard Selman]

Doug thanks for that. So best prop was 23"P x 14 1/2" D - bigger diameter than mine hopefully confirming what I think is wrong. Might take a punt on that. I see that Yamaha also do the performance XL range with a 23" P x 14 5/8 D prop. What was interesting was that the 21, 22, 23 and 24 P props all ended up with the same top speed within a knot.

Richard

[Roy Smith]

Not 100% sure but some of the Yamaha props skip a size
i/e 21" then 23" as we where not pulling max rev Scorpion advised us to send the prop off and have it resizes to a 22", but where do you stop.
What fuel consumption do you get lts to miles

[Richard Selman]

Display say 3.9 Nm/uk gallon I think. Not confirmed this yet by any other measurements. I have a petrol station at the bottom of my drive. I just pause as I pass by and Sandy fills it up - it just goes on my account. Its all kind of magic and painless until the bill comes in at the end of the month
Richard

[Robbie Diesel]

Hi Richard,

I Don't have a Yamaha f250 but I do have a Yamaha 300hpdi on a 7.5 Metre Rib and have had it 4 years now and tested many props back to back. I Have tried the Yamaha Pro-Series, the Yamaha Salwater Series 2, Ballistic, Mercury Laser 2, Mercury Rev 4, and the Mercury Tempest Plus props. The Mercury Tempest Plus (14 5/8" Diameter) is by far the best prop i've tried on the Boat. Yamaha Pro-Series comes a close second but the Tempest gave marginally better numbers all round. Also check the engine height in relation to the keel as mine was mounted far too low from new and this can be a common problem. Yamahas like to run high Propping a boat right can be an expensive process but well worth while. I've gained nearly 10 knots since i got the boat new plus made big savings at cruise rpms aswell. Good Luck Man

Regards

Robbie

[Richard Selman]

Thanks for that Robbie. The anti-ventilation plate is about 30 mm above the base of the transom. The vipermax has a big planing pad and therefore the engine must be sitting higher relative to the keel than for a conventional hulled RIB.

How would I know if the engine was too low? By raising the engine, you must reduce the leg drag at the risk of reducing prop thrust by running it in less dense /streamlined water (?). Since I am achieving max revs anyway, surely I would only see a benefit if the prop was changed as well.

Two other bits of information: trimming out any does not increase the speed. The boat only feels right when fully trimmed in. Also, I experience a lot of torque. As well as a 1.5" offset on the main. The boat would only run level with me + 45 kg of auxilliary on the starboard side (I calculate that this is equivalent to a main engine offset of at least 3". The boat runs perfectly level now but the fact that I had to shift so much weight to starboard must be telling me something about the prop selection or positioning of the main engine not being right.

Richard