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

[julians]

If its down to gear ratios, doesnt that impy that the boat with the more powerful engine,but lower top speed is therefore underpropped?

And therefore must also be the faster accelerating of the two.

 

[ari]

Yes.

 

[zt260]

The kad44 is probably a dpe outdrive and the kad300 will be a dpg outdrive they should both be on stainless props either c4 or c5 depending what performance you want either acceleration or top end.what props are you both running
Regds ash

 

[Whitelighter]

The kad44 is probably a dpe outdrive and the kad300 will be a dpg outdrive they should both be on stainless props either c4 or c5 depending what performance you want either acceleration or top end.what props are you both running
Regds ash

I think on the Rodman they will be on a shaft drive - So technically KAMD motors....

 

[zt260]

I wondered that but took a flyer with the op calling then kad

Regds ash

 

[James L]

If it's shaft drive, could the props have been altered without changing the markings?

 

[sorabain]

maybe your friend has nicked your boat with the higher HP and swapped all the stickers around so you're none the wiser.

 

[Latestarter1]

Boat speed is all about weight plain and simple.

Two identical boats have never been built.

Power has far less impact on performance than weight.

No clue about details of VP power units other than the limitations of the Bosch fuel pump used, however mechanical engines have a tolerance band usually plus minus 5% but can be as high as 7%., perhaps VP guys have the data.

Transmission reduction ratio has nothing to do with rated engine speed.

 

[MarieK]

Firstly let me thankyou all for the response, very helpful. Let me answer some of the questions posed so far.

1. Its shafts not drives, apologies for throwing people by calling them KAD
2. Props are same manufacturer with same stamps, admittedly ive never had them off and measured nor swapped them, should look at this.
3. I need to check the gearboxes but pretty sure they are the same.
4. My boat is heavier by 250kg when in the slings
5. Never tried an acceleration test but I have no reason to believe my boat is stuggling, no excess smoke and achieves wot easily.

Let me go check gearboxes and get back to the forum

 

[ari]

Boat speed is all about weight plain and simple.

Two identical boats have never been built.

Power has far less impact on performance than weight.

No clue about details of VP power units other than the limitations of the Bosch fuel pump used, however mechanical engines have a tolerance band usually plus minus 5% but can be as high as 7%., perhaps VP guys have the data.

Transmission reduction ratio has nothing to do with rated engine speed.

Agree that additional weight would slow a boat down, but you WOULD see a reduction in revs.

If you have a boat that does 30 knots at 3,000rpm and you weigh it down until it is only doing 25 knots it will not be still hitting 3,000rpm at that speed.

 

[ari]

5. Never tried an acceleration test but I have no reason to believe my boat is stuggling, no excess smoke and achieves wot easily.

You wouldn't see any of those things - quite the reverse in fact. If your boat is effectively under propped (be that literally under propped or via a gearbox being effectively in a lower gear) you boat would in fact 'struggle' less, be less likely to produce excess smoke and hit max RPM even more easily.

To re-use my analogy above. Imagine two identical Ford Focus cars on a flat runway at 60mph when you both go to WOT (wide open throttle). You are in fourth gear, your mate in fifth. You will out accelerate him and hit max revs more quickly and easily, but he will be travelling faster when he hits max revs.

So the fact that your boat isn't struggling, producing smoke and hitting max revs is not indicative that the 'gearing' is correct - the fact that you are getting five knots less at WOT and (most importantly) the same revs suggests that something along the drive-train you is 'gearing you down', putting you (effectively) in fourth instead of fifth.

Swapping props with your mate would be the very first thing to try as it is quick and (relatively) cheap.

 

[ari]

4. My boat is heavier by 250kg when in the slings

Sorry, just on this point too - and this is as a result of a very quick Google so someone please correct me if I am wrong.

250kg is the weight of about 240 litres of fuel (about 50 gallons).

The fuel capacity of a Rodman 900 is about 600 litres (apparently, according to a quick Google).

You certainly don't gain five knots (or anything like) running with a half a tank compared to running with a full tank so I don't think that will make a material difference to performance between the two boats. (And even if you did, you'd see a corresponding change in RPM).

 

[MarieK]

OK I understand the thinking here - swapping props is the next thing I will try. It seems to be the only logical explanation if the gearboxes are same ratio

You wouldn't see any of those things - quite the reverse in fact. If your boat is effectively under propped (be that literally under propped or via a gearbox being effectively in a lower gear) you boat would in fact 'struggle' less, be less likely to produce excess smoke and hit max RPM even more easily.

To re-use my analogy above. Imagine two identical Ford Focus cars on a flat runway at 60mph when you both go to WOT (wide open throttle). You are in fourth gear, your mate in fifth. You will out accelerate him and hit max revs more quickly and easily, but he will be travelling faster when he hits max revs.

So the fact that your boat isn't struggling, producing smoke and hitting max revs is not indicative that the 'gearing' is correct - the fact that you are getting five knots less at WOT and (most importantly) the same revs suggests that something along the drive-train you is 'gearing you down', putting you (effectively) in fourth instead of fifth.

Swapping props with your mate would be the very first thing to try as it is quick and (relatively) cheap.

 

[ari]

Good luck, and let us know how you get on.

 

[neale]

I know that the two engines are basically the same, power output aside, which means there is no reason for what I am about to suggest. But it hasn't been mentioned and, as no two boats are ever really identical, it's a remote possibility.

Could the shaft angle be different? A shallower shaft angle will provide better performance in terms of less slippage.

Just chucking it into the pot

BTW: diesel weighs about 0.9kg per litre, so 250kg would equal about 280 litres. Not that it changes anything

 

[MapisM]

as no two boats are ever really identical, it's a remote possibility.
Could the shaft angle be different? A shallower shaft angle will provide better performance in terms of less slippage.

Nope, that possibility is not even remote, imho.
The fact that the two boats are the same is a sort of red herring, in this case.
The OP concern would be equally valid even if the two boats were completely different.
I mean, according to the initial assumptions, what we have got here are two boats which, when pushed by exactly the same props, spinning exactly at the same revs, have a top speed difference of 5 knots.
Now, if that would be true, the slower boat should have - BY DEFINITION - a higher prop slip.
Higher exactly as much as necessary to justify the whole speed difference.

Unfortunately, we are missing two critical numbers here, to quantify the prop slip: the gear ratio and the prop pitch.
But assuming that the faster boat has a slip of 10%, in the slower boat it should be around 25% - or even more, if the faster boat would have a higher slip, which could well be the case. Anyhow, 25% is already a number that doesn't pass the sanity check for any boat.
And that's without even considering the two identical hulls and the irrelevant weight difference that was previously mentioned, which would lead to expect an almost unmeasurable difference in the prop slip!

Naah, either the gear ratio or the prop pitch (or both) are bound to be different.
There's no other logical explanation, based on the OP premises.

 

[Croftie]

Surely shaft angle could be a contributory factor. If one boat has a steeper angle more energy is going to lift and therefore less to forward motion. Obviously if the boat is lifted more then less drag so perhaps a slight cancel out.

Two harrier jump jets flying side by side. Exactly the same thrust. If one is vectoring at a slightly different angle one will go slower (and climb) than the other one.

 

[ari]

Yes it could be, in theory. But two identical production boats? How much difference in shaft angle could there possibly be? They'd have to put the engines of the more powerful boat in the saloon to cant the shafts down so much that you'd lose five knots despite having more power.

 

[Latestarter1]

Agree that additional weight would slow a boat down, but you WOULD see a reduction in revs.

If you have a boat that does 30 knots at 3,000rpm and you weigh it down until it is only doing 25 knots it will not be still hitting 3,000rpm at that speed.

Ari,

There are still many factors not being considered, for example tacho calibration.

In addition I have a brig green hole on the engine front as I choose to have nothing to do with VP engines, let alone differences between similar models.

In addition I have had props scanned, both from same manufacturer and whilst they looked the same, scan and vessel performance (two knots) was different.

Let me try things arse backwards. Give me accurate slinging weight, water line length, propeller diameter, gearbox manufacturer and I will come up with some numbers.

However

 

[A_8]

I think a taco calibration is a good place to start, if you have the VDO units they need to be set to match the signals if I remember correctly.
The KAD44 and the 300 are more or less the same engine, the 300 give another 25-30hp vs the 44 which is why what the op describes makes no sense. The Kad 300 boat should be the faster one with a slightly higher pitched prop or different gear ratio. If the kad300 boat is heavier etc it should also get lower revs, there just is no way round it.
This is a production boat so no individual specs unless of course some ex owner did something.
Another thing you could do is get the original specs on prop size, gearbox etc from Rodman and compare that to what you have.