Two boats virtually identical - one of them 5 knots faster at WOT

[MarieK]

This has been an annoyance to me over the last few years. My friend and I both have Rodman 900's. Mine is a 2002 with twin Kad300, his is a 2001 with Kad44. On the older boat with Kad44 wot delivers 31 knots, on my boat with Kad300 wot delivers 26 knots. This was measured side by side numerous times in same sea state etc and most recently after both relaunched after a clean so no extra fouling, same water onboard, same fuel.

Both boats achieve 3800-3900 rpm at wot, have exact same hulls, have exact same stern gear with identical props. Both have same rope cutters.

There are some differences, I have a bowthruster, bennett trim tabs and coppercoat which other boat does not but other than that they are identical.

Any thoughts from the forum on what could be going on? 1-2 knots I could explain by setup differences but 5 knots more from an identical boat with smaller engines is difficult to understand...

Its also apparent at cruising rpm - 3000 rpm gives me 20 knots and him 23 knots

All I can think of is something happened in the hull between his build and mine but standing them side by side in the yard its impossible to see a difference.

 

[Deleted User YDKXO]

Tricky one that because the difference is so large. Assuming that the distribution of component weights is the same (do you notice any difference in running attitude?), it can only be down to weight and/or engine power. Have you had your engines checked to see whether they are in good condition and developing the power they should under load? It would also be interesting to weigh both boats the next time they are lifted to see whether Rodman increased the lay up weight on the later boat. On that subject have you asked Rodman or their UK dealer why this difference should exist?

 

[MapisM]

Hang on a minute, there MUST be a difference somewhere, either in the gear ratio or prop pitch, at a guess.
I mean, forget the hull, the engine power, the weight - forget everything.
If two identical propellers, spinning exactly at the same rpm, would push one boat at 26 knots and another one at 31 knots - regardless (!) of the fact that the two boats are identical - the only explanation would be a prop slip difference between the two boats.
Not saying it's impossible in principle, but based on your numbers your prop slip should be 15% or so HIGHER than in your mate boat, and that simply doesn't stand to reason!

 

[KINGFISHER 8]

Weight of the engines? ... Props?

 

[septiclecky]

The is an earlier posting about these engines

http://www.ybw.com/forums/showthread.php?187884-KAD-300-vs-KAD-44

 

[KevB]

Trim tabs being down could cause drag themselves and also push the hull down so more surface area in the water causing more drag?

 

[Deleted User YDKXO]

Hang on a minute, there MUST be a difference somewhere, either in the gear ratio or prop pitch, at a guess.
I mean, forget the hull, the engine power, the weight - forget everything.

Yes having thought about it, I agree with that. If the engines were down on power and all other things being equal, the engines wouldn't make their max rpm. Assuming the OP is right and the props are exactly the same, that only leaves the gear ratios. With regard to the props, the OP could try swapping them with the faster boat just to check that there is in fact no difference

 

[Whitelighter]

Surely if the props are genuinely the same and both biats making wot then it must be gearbox or some sort of induced drag.
I'd be suprises if tabs can knock 5 knots off. I know fouling can.

My concern is how much harder the engine must be working if it's drag.

What about acceleration? Whose is faster if you just nail the throttle from a standing?

 

[KevB]

S
I'd be suprises if tabs can knock 5 knots off. I know fouling can.

maybe not just the drag of the tabs but the angle of attack they are putting the hull in?

 

[ari]

Yes having thought about it, I agree with that. If the engines were down on power and all other things being equal, the engines wouldn't make their max rpm. Assuming the OP is right and the props are exactly the same, that only leaves the gear ratios. With regard to the props, the OP could try swapping them with the faster boat just to check that there is in fact no difference

Precisely! All these theories about trim tabs and weight miss the point that if something physical was slowing the boat by five knots it would be reaching the corresponding revs for five knots slower.

If you take a boat that does 30 knots at (say) 3,000rpm and drag it back to 25 knots due to excess weight, drag, or whatever then the revs would be 2,500rpm (give or take, allowing for a little prop slip).

The boat is hitting the same max revs same as the friends boat so if the revs are the same but the speed is 5 knots shy then there has to be a difference in effective gearing somewhere, either the props or the gearboxes or somewhere in the drivetrain.

 

[scottie]

Weight weight weight It is its place to check with load cell if you want to be ultra sure

Are the drives identical ratios

 

[Chris_d]

Surely if one has a more powerful engine it shouldn't have an identical prop!

 

[A_8]

As have been said it does not make sense, identical boats with the same transmission and props can't differ in speed given the same rpm as you describe so something is different. Outdrives have different gear ratios, have you checked that?
I know there are trim kits available for the 44, don't know how much power they give but perhaps that's what's going on?
If your boat was heavier, had more drag, your engines have lost power and everything else equal you would get lower rpm with your lower speed....unless your props are slipping but it's unlikely they all do so then you would have a rpm mismatch between the engines.

 

[Croftie]

If you take a boat that does 30 knots at (say) 3,000rpm and drag it back to 25 knots due to excess weight, drag, or whatever then the revs would be 2,500rpm (give or take, allowing for a little prop slip).

Agreed if it was a car were there is no slip between road and drive train but surely a prop in the water does slip, it's not 100% efficient.

If weight and drag (Trimtabs) had no effect then any boat with say 300bhp that reached 3000rpm at WOT with the same gearing and prop they would travel at the same speed? 30ft 3 ton and 35ft 4 ton, would they go at the same speed?

 

[ari]

Agreed if it was a car were there is no slip between road and drive train but surely a prop in the water does slip, it's not 100% efficient.

If weight and drag (Trimtabs) had no effect then any boat with say 300bhp that reached 3000rpm at WOT with the same gearing and prop they would travel at the same speed? 30ft 3 ton and 35ft 4 ton, would they go at the same speed?

No they wouldn't go at the same speed, the heavier boat would go slower. With correspondingly lower revs.

That's the point. Find a boat that does 30 knots at 3,000rpm at WOT (assuming it isn't hitting a rev limiter), load it down with a couple of tons of lead or drag a small parachute behind it and of course it won't hit 30 knots any more. But it won't be hitting 3,000rpm either.

You see it all the time with boats at the end of the season that are down on speed due to hull growth. The revs don't stay the same as when it is clean, they drop too.

 

[springer]

Initial data does not add up, KAD44P, is listed to produce 182Kw (248Hp) at the props at 3800 engine rpm, KAD300 is 200Kw, (272Hp) at the same rpm. Therefore where is the 24Hp per engine disappearing to?? KAD300 boat will always be faster if boats are indeed identical. It is also unlikely, but possible, that these boats would have identical propeller sets, it would certainly be a test to swap as suggested previously. If engines are all performing at peak efficiency then another significant factor must differ. Question for the panel----if one of the pair of DP props is slipping its hub at peak power, will the Volvo EDC / electronic governor hide this fact by limiting the light load engine to 3800 rpm??
Springer

 

[Lon nan Gruagach]

No they wouldn't go at the same speed, the heavier boat would go slower. With correspondingly lower revs.

That's the point. Find a boat that does 30 knots at 3,000rpm at WOT (assuming it isn't hitting a rev limiter), load it down with a couple of tons of lead or drag a small parachute behind it and of course it won't hit 30 knots any more. But it won't be hitting 3,000rpm either.

You see it all the time with boats at the end of the season that are down on speed due to hull growth. The revs don't stay the same as when it is clean, they drop too.

That might be the point though, a limiter. From the OP

Its also apparent at cruising rpm - 3000 rpm gives me 20 knots and him 23 knots

Here both throttles are set for 3,000 rpm (by a human limiter at least). Same props, same RPM, different speed. So, the only other thing that is relevant is what is touching the water. If the hulls are identical then attitude, weight and friction are the only factors.

 

[Deleted User YDKXO]

Another suggestion to check. Do both of these engines have management systems giving instantaneous fuel consumption data? If the slower boat is consuming significantly less fuel at WOT under load then that suggests that the engines in the slower boat are not developing full power and that IMHO points to a gearing or prop difference

 

[KevB]

It does seem that there are two ratio option for the kamd 300 and 44's @ 1.96:1 or 2.62:1

https://www.marinepartseurope.com/en/e-volvo-penta-553-KAD300-A.aspx

 

[ari]

It does seem that there are two ratio option for the kamd 300 and 44's @ 1.96:1 or 2.62:1

https://www.marinepartseurope.com/en/e-volvo-penta-553-KAD300-A.aspx

Gear ratios would seem the obvious place to look to me. There is 3 knots difference at 3,000 and 5 knots at WOT with the same props and hull. Bit like two identical cars side by side on a runway only one is in fourth gear and the other fifth gear.