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

[ari]

How does tachos being out affect max speed? To take it to the extreme, say they are both really massively out and reading 5,000rpm (or conversely 500rpm) at WOT, full throttle is still full throttle and the max speed is still the max speed (which is 5 knots down on his mates identical but slightly less powerful boat).

And are they both likely to be out by exactly the same amount? (The OP doesn't mention differing revs at WOT).

 

[A_8]

I was thinking something could be wrong with the kamd300 engines, not producing the power they are supposed to and the taco's showing more revs than they actually run at.

 

[Croftie]

How does tachos being out affect max speed? To take it to the extreme, say they are both really massively out and reading 5,000rpm (or conversely 500rpm) at WOT, full throttle is still full throttle and the max speed is still the max speed (which is 5 knots down on his mates identical but slightly less powerful boat).

And are they both likely to be out by exactly the same amount? (The OP doesn't mention differing revs at WOT).

In the OP he did imply it was elsewhere in the rev range as well as WOT

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

He also says

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.

Bow thruster? water turbulence affecting water flow to prop? more weight at front - Bowthruster + batteries = bow down.
We also know his boat is heavier (250kg from memory) so must be lower in water = greater wetted area = resistance.

 

[grumpy_o_g]

It's no help at all but I keep imagining you and your family sneaking in one night when the two boats are nearly, switching berths with his boat, swapping all your belongings over, carefully removing the names and rewriting them on the other boat and then leaving him wondering why his boat is now 5 knots slower...

 

[Jim@sea]

With both boats tied up in a marina could you put a spirit level on the prop shaft and compare it with what it reads on the other boat. It could be that with different engines the propshaft on one boat may be at a slightly different angle so that the propeller is turning on the slower boat as a slightly disadvantaged angle.

 

[Latestarter1]

In the OP he did imply it was elsewhere in the rev range as well as WOT



He also says


Bow thruster? water turbulence affecting water flow to prop? more weight at front - Bowthruster + batteries = bow down.
We also know his boat is heavier (250kg from memory) so must be lower in water = greater wetted area = resistance.

Why do folks make life so complicated........Vessel speed is a characteristic of weight nothing else, bow-thuster tunnel is clear of the water at planing speed so has zero effect on laminar flow. Bit like the shaft angle stuff all irrelevant.

We have no clue yet as to as displacement therefore how large a percentage 250 kg is of total displacement.

As I mentioned previously two identical vessels have never been built.

Alan Burnard (Designer of Fairey power boats) kept a log of owners changes to specification during build and constantly warned about the potential impact on speed which he correlated with sea trial data.

 

[ari]

In the OP he did imply it was elsewhere in the rev range as well as WOT






Bow thruster? water turbulence affecting water flow to prop? more weight at front - Bowthruster + batteries = bow down.
We also know his boat is heavier (250kg from memory) so must be lower in water = greater wetted area = resistance.

It is elsewhere in the rev range yes, but the question remains, how would incorrectly reading tachos slow the boat by five knots at WOT (assuming everything else mechanically is the same)? They might show an erroneous reading yes, but how are they physically slowing down the boat?

Bow thruster - plenty of boats are available with or without bow thrusters, indeed people have had them retro fitted. They have virtually no effect on performance.

The weight issue has been discussed - it is equivalent to about half a tank of fuel. How much speed difference does a 30 knot boat boat gain or lose between a full tank and half a tank? I doubt it is measurable, 1/2 knot at most?

 

[ari]

Why do folks make life so complicated........Vessel speed is a characteristic of weight nothing else, bow-thuster tunnel is clear of the water at planing speed so has zero effect on laminar flow. Bit like the shaft angle stuff all irrelevant.

We have no clue yet as to as displacement therefore how large a percentage 250 kg is of total displacement.

As I mentioned previously two identical vessels have never been built.

Alan Burnard (Designer of Fairey power boats) kept a log of owners changes to specification during build and constantly warned about the potential impact on speed which he correlated with sea trial data.

If it were the 250kg weight difference he could solve it by running with half a tank of fuel instead of a full tank and be back up to 31 knots.

 

[MapisM]

Vessel speed is a characteristic of weight nothing else

LS, just to clarify my previous statement about the "irrelevant weight difference":
I wholeheartedly agree that the weight (and its distribution) is of paramount importance in the performance of planing boats.
In my last lake boat, each additional passenger reduced the max speed of just about 1 knot.
But that was a 2.5T boat akin to a scalded cat, capable of 62 knots.
And even then, the speed difference was reflected in a lower max RPM, rather than a higher prop slip, anyway!

Here we are talking of a sedated boat in the 7T ballpark, designed for cruising comfortably at 20 kts or so, ffs.
Imho, there's absolutely no way that a weight difference of less than half of its tank capacity could reduce the max speed by 5 knots.
And even if it would, why should that be reflected in such a crazy prop slip (assuming - but again I don't think it is the case - that the props are exactly the same and are spinning at the same speed), rather than a lower rpm?

 

[Latestarter1]

Mapis,

This stuff is like fisherman's stories therefore I do not take too much notice of absolute #'s quoted.

All a matter of digging away until you hit the relevant nugget.

IF I had done installation review and sea trial on both vessels and run both over measured mile under exactly the same sea/wind conditions then we would have some REAL data to work with otherwise everything is a SWAG (Scientific Wild Assed Guess).

Once I have some numbers we have something to manipulate and decent basis for discussion.

 

[neale]

I also think the 250kg weight difference is a red herring. Remember the op's boat has extra power so to all intent and purposes it should still go faster, or at least as fast, even with an extra 250kg.

Now if two identical boats are fitted with engines that both rev to 3800 and are both fitted with the same gearbox ratio and same prop then the extra power will make no difference. The boats will go at the same speed. The higher powered boat might get there a bit quicker I guess.

So if you are going to take advantage of that extra power, and the revs are going to stay the same, you will have to change gear ratio or prop size. There are only two gear ratios possible AFAIK 1.96 and 2.62. If we assume zero slippage (I know it's not possible, but bear with me) and a 20" pitch (another assumption as I don't think we know the actual size) you can work out that at 3800 rpm the 1.96 equipped boat will be traveling at 31.9 knots while the 2.62 equipped boat will only be hitting 23.9 knots. Stick a 22" pitch prop on the latter and assuming it can still pull 3800rpm it will now be traveling at 26 knots. What I am trying to say is that there has to be a difference between these two boats, and regardless of whether they have the same or different gear ratios, I don't think they should have the same prop size. Same gear ratios and same prop size equals same speed. Different gear ratio and same prop size equals one much slower boat.

My thoughts are that they should probably have the same gear ratios, but the KAMD300 boat should have a slightly bigger prop in terms of pitch. With a 1.96:1 you should get 33 knots at 3800 with a 21" pitch, against 31 knots for a 20" (all not taking into account slippage of course)

But............if the OP's boat is under propped, I would expect it to rev higher, and the OP is telling us that both boats hit 3800-3900 WOT. Is it possible that the KAMD300 is governed to not exceed its maximum rated WOT?

 

[Colvic Watson]

How does tachos being out affect max speed?

If the tacho was part of the maximum rev limiter system, then could a faulty tacho hold the actual maximum revs down?

 

[ari]

If the tacho was part of the maximum rev limiter system, then could a faulty tacho hold the actual maximum revs down?

We would need a marine Volvo expert to tell us that but I'd be surprised if a 13 year old KAMD300 is electronically controlled to that degree, they'd only just got around to bolting electronic throttle/shifters onto them back then. I suspect it would be mostly mechanical.

And you'd still need both tacho senders to be inaccurate to the same degree otherwise you'd have a rev mismatch.

I'm hanging my hat on incorrect props or gearbox ratios effectively gearing the boat down (like operating permanently in fourth gear rather than fifth) but I'll be fascinated to find out the real answer.

 

[Croftie]

We really need confirmation that both are using the same gearbox ratio etc. There are plenty of very cheap laser RPM meters on Ebay.

 

[MarieK]

LS if you could list the numbers you need I will happily try and get them as accurately as possible, any help on this greatly appreciated.

Mapis,

This stuff is like fisherman's stories therefore I do not take too much notice of absolute #'s quoted.

All a matter of digging away until you hit the relevant nugget.

IF I had done installation review and sea trial on both vessels and run both over measured mile under exactly the same sea/wind conditions then we would have some REAL data to work with otherwise everything is a SWAG (Scientific Wild Assed Guess).

Once I have some numbers we have something to manipulate and decent basis for discussion.

 

[MapisM]

LS if you could list the numbers you need I will happily try and get them as accurately as possible, any help on this greatly appreciated.

MK, I think that the number LS was referring to are just the usual stuff which make a proper sea trial meaningful.
I mean, boat displacement, gear ratio, propeller specs, average of max rpm and speed achieved going back and forth (to compensate any potential wind/current induced error) through the same stretch of (calm) waters.
But for some of these #s, like weight, rpm and prop pitch, it's not trivial to get an accurate measurement.
Weight and prop could be different from the nominal numbers that you can get from the builder, and even the tacho could be not accurate enough.
For speed, nowadays the one you get from GPS is good enough, provided that you measure it in opposite runs.

Anyway, it would be interesting if you could check and post the gear ratio and the prop pitch (at least the nominal one) of the two boats, to start with.
You already mentioned that the props have the same stamps, but are you sure that the pitch is included in that stamp?
Some props have more than one stamp, and the most useful one can be in silly places.
Besides, don't assume that the gear ratio is the same just because the gearboxes look identical: most gearboxes are available with different ratios, and the only external difference are the numbers stamped in a small steel plate attached to them.

 

[Latestarter1]

Before my wee brain implodes with more unbelievable complexity being introduced here and absence so far of real data have taken at stab at some numbers, which I hope appear meaningful.

Took a look at Rodman 900 hull form and some weights of vessels for sale with Volvo Penta engines which seem to vary between 6.75 and 7.5 tonnes which seems mad heavy to me for a 30ft vessel.

Transmission from spec sheet is ZF63A, these little Volvo Pentas have simply hopeless torque rise due to to oversquare cylinder dimensions however I will be brave and use the ZF 1.563:1 transmission ratio. Propeller diameter is a SWAG, lets say 18 inches. LWL is a bit academic but 28 ft would be reasonable.

If I was to wring the neck of a pair of VP KAMD 300 they have 548 hp to offer at rated speed of 3,800 rpm.

Using 7.5 tonnes and a 18x21.5 three bladed propeller at 3,800 using 2.7 exponent we can absorb 100% of the engine output giving us 32.5 knots.

Looking at data for the Rodmans for sale the 4HLA 240 powered vessels are quoted as being good for 28 knots so hull does seem sensitive to weight.

Let's see how my SWAG lines up with the real data.

 

[ari]

You seem convinced that this is all down to differing weights between the two boats.

We know that there is a 250kg weight difference between the two boats. So are you saying that if three average weight blokes get on his mates 30ft twin diesel boat (so it weighs the same as the OPs boat), you believe it will drop from 31 knots to 26 knots? (Less, actually since it has less powerful engines)? And that despite the five knot loss in speed the revs will remain at 3,800-3,900rpm?

 

[Latestarter1]

You seem convinced that this is all down to differing weights between the two boats.

We know that there is a 250kg weight difference between the two boats. So are you saying that if three average weight blokes get on his mates 30ft twin diesel boat (so it weighs the same as the OPs boat), you believe it will drop from 31 knots to 26 knots? (Less, actually since it has less powerful engines)? And that despite the five knot loss in speed the revs will remain at 3,800-3,900rpm?

I am not making any assumptions yet............Lets try and get a reasonably accurate model of the propeller demand curve for OP's vessel, I hate banging on about this but here we go again 'propellers move boats' the engines merely turn them.

Do you have any issues with the SWAG numbers I have created so far?

 

[Sandy Bottom]

MarieK - when you get a minute I would be interested to hear what device lifted each boat out to give a 250kg difference?
Was it a crane or a boat hoist?
What was the capacity (i.e. were you a +/- 7.5 boat on a 500 ton crane or a 70 ton hoist etc.).
How old was the lifting machine, did he have a calibrated digital weight readout or was it a conversion from hydraulic pressure on the lift motors of an old hoist?

Ta.