Top speed

[John Hughes]

Before I get in a real muddle by taking all the advice offered about prop size/pitch, height of engine relative to hull etc. should I be looking at something more fundamental?

Boat will only pull 4500-4750 rpm one up or fully loaded with fuel, picnic and 9 people. Also, skiers who ought to know better struggle to get up easily.
The hull is I think shallow V, and top speed 30-35mph (GPS) Bit dissapointing! The handbook says engine should rev 5-6000rpm. and the whole thing came as a ready to go package from the Valiant factory who sell hundreds of the things, so i would have thought they have installed it properly and that the prop is the correct one? I don't know the prop size, but it's alloy 3 blades. Perhaps this as fast as it's meant to go and I just specified the wrong engine? At the time it was the biggest 4 st efi that Mercury did, and by far the lowest price per h.p.
One option is to not use the gps, and then i'll probably stop worrying about it..

[Brian]

I am certainly no expert, but I think you should borrow and try a smaller prop. The next size down should do it. You should be able to reach the manufacturers max. revs at WOT if you have the correct prop. A smaller prop would also give you better acceleration for water-skiing.

[Daniel]

Take your point about you being one of 100's and surely everyone else has not got the same problem - BUT, everything points to a prop too big for the set-up.

Try and find a local marine dealer who will lend you a stainless prop (coz they are better than alloy - if you are changing then go stainless on an engine of this size) before you buy!

I'm guessing a bit, but 6m boat, 115hp, about a 21-22" prop, maybe 20" but you can't always get exactly what you want. The Laser II stainless range only come in even numbers for v4's and odd's for v6's.

Not sure if this rule applies to 4strokes, but is standard accross yam/merc/mariner v4 and v6's. For your info i use a 22" or 24" depending upon skiing or cruising, the 24" is 5600 WOT trimmed right up.

[TIPS]

I have a Yammy 115 EFI 4-Stroke, im lead to believe that it is a very similar engine to the Merc. The engine is mounted on a Avon Adventure 5.6. At WOT it will do about 6200 revs. This is with a 13" diametre, 19" pitch ally 3 blade prop. Gives us a top speed of about 40 knots, with about 4 people in. Before you do anything else, i suggest that you try and find out what size prop it is that you currently have, it should be written on there somewhere (13-19-K) or something similar. Give Valiant a ring if u can't find it. Then go from there.

Unlike 2 Strokes, even changing the pitch up or down by one on a 4-stroke can radically change the revs.

Mark

[dimitris]

Hi,

The following are some formulas to make an estimation of the engine and prop size for your boat.

They are not 100% exact formulas due to differences in hull design, boat length etc., but they should give a close figure.

In order to estimate the required engine power apply the following formula

HP= (Knots)^2 * (Kg) / 13500 Where

HP = The suggested engine size
Knots = The required maximum speed in knots that we want to achieve raised in the square power
Kg = The total weight of the rib including all the equipment (Boat + Engine + Fuel + passengers + everything else)

As an example if the boat weights 1,150 Kilos (Rib 565kg + Outboard 200Kg + Aux. engine 35Kg + Fuel 200Kg + Two passengers 150Kg) and we want to achieve a top speed of 40 Knots then the horsepower required is approximately
HP = 40^2 X 1150 / 13500 = 136.30 HP , which means an outboard in the range of 135HP – 140HP

Once the proper engine size is selected for the required top speed (in our example 40knots) then the appropriate propeller pitch can be calculated using the following formula

Pitch = (Knots) X (RR) X (1350) / (RPM) Where

Pitch = the required pitch in inches
Knots = The top speed that we want to achieve
RR = The gear ratio of the engine
RPM = The maximum RPM (WOT) of the engine

Now, continuing our example for the above rib (1150 kilos weight) and the required top speed of 40 knots, if we select an 140HP outboard (let’s say a four stroke suzuki 140DF with a gear ratio of 2.38 and max RMP (WOT) 6200), the proper pitch is calculated as following

Pitch = 40 × 2.38 × 1350 / 6200 = 20.73'' rounded at 21’’ (inches).

The next step is to deal with the diameter of the propeller. Most outboard manufactures suggest one or two propeller diameters for a given outboard (usually in the range of 12 1/2'' to 15 1/2'' inches.

Since everything in life is a trade off, choose the diameter using the following criteria

Shortest Diameter means your engine will rev easier under heavy loads, reaching easier WOT, achieving thus top speed. The choice of the shortest Diameter is usually suggested for boats under 1000 kilos of weight (including rib, engine, passengers etc).

Longer Diameter means more thrust during planning, better handling at mid range revs, more economy at cruising speed, but more difficult to reach MAX RPM and thus more difficult to achieve top speed

If you go for the longer available diameter, then the calculated pitch should be reduced in the immediate smaller size. Ex from 21’’ to 19’’.

Following our example, for the four stroke 140DF HP , Suzuki suggests propellers of diameter 13 ½’’ and 14’’.

According to our calculated pitch of 21’’ you should go for the shortest diameter (13 ½ ‘’ ) if top speed is the criterion.

If you opt for easier planning, handling in mid range revs, fuel economy in cruising speed) then you should go for the longer diameter (14’’) lowering as I mentioned above, the pitch by 1 step to 19’’.

Using a Pitch of 19’’ instead of the calculated 21’’ pitch will result in reaching the required 40 knots at 300 RPMs above the 6200RPMs that we used in our formula, at 6500 RPMs.

In all cases you should be able to achieve revs close to the maximum RPMs of your engine under the normal load of your boat.

I hope this helps .


Regards


Dimitris

[Charles]

That must be the longest post ever!

[Viking]

and technically enlightening

[dimitris]

Well charles,

J.K pays me by the length of the message !
Do you mean that you don't get paid for your postings ?


Dimitris

[batfalcon]

The last reply exposed all us Greeks !!!!

[John Hughes]

It's all double dutch to me or rather double greek!
Seriously, thanks for all the advice. Unless I can do all the calcs. and be a bit more scientific, I suspect i will just get a smaller prop and try it, At least nobody's said thats it, it won't go any faster whatever I do.