Displacement hull speed

[alexhibbert]

Hi all,

I have a 7.5m displacement TELB hull with a DV48 Bukh diesel. (48hp turbo 3-cyl)

The SOLAS requirement is to achieve a steady 6 knots, and this is what I get at mid revs. Up top there's not much more speed - maybe a knot or two.

It's not a tiny engine, and I'm aware there are complex methods to calculate a max speed for a hull, but is there a method to extract more pace? The propeller is as original, and I'm not sure whether a different design could get the boat up to nearer 10 knots without being up the top of the power curve and wasting fuel.

Thanks.

 

[PCUK]

I don't know what a telb is but generally huge trim tabs will help.

 

[alexhibbert]

I don't know what a telb is but generally huge trim tabs will help.

A totally enclosed lifebat - Captain Phillips-esque.

 

[TLouth7]

You won't get to 10kt in that boat - it won't plane. Speed simply isn't an important design parameter for ships' lifeboats.

You can minimise the fuel burnt for any given speed by ensuring your hull is well faired and free of fouling.

 

[alexhibbert]

You won't get to 10kt in that boat - it won't plane. Speed simply isn't an important design parameter for ships' lifeboats.

You can minimise the fuel burnt for any given speed by ensuring your hull is well faired and free of fouling.

Thanks. That's what I thought. They are designed to simply stay safe and under control for 24hrs. A few others have suggested that 10knots might be possible, hence my question about the prop. I certainly would expect to plane in a TELB! Just perhaps eeking out three or four more knots without going crazy high with the revs and wasting fuel.

Below the waterline it's currently just gelcoat (of course they are designed to be out of the water 99% of their lives), and I'll antifoul that before a proper trip.

 

[madabouttheboat]

You'll reach your hull speed, which is circa 6 knots, very efficiently. After that, you may be able to eek out another knot or so, but that extra know will probably double your fuel consumption. Cannot imagine ever getting to 10 knots in a boat of the size and style I'm imagining from your description, particularly with only 48hp. Maybe with a few hundred HP you might get near it, but fuel will be multiple times what you burn at 6 knots.

 

[Slowboat35]

There is nothing complex about calculating hull speed.
In a displacement hull it is approximately 1.5 times square root of waterline length in feet. Adding more power simply has no effect unless there is sufficient to force the hull into a semi-displacement mode or onto the plane.
Assuming your waterline is 7m, ie 22 ft, the hull speed is 4.7 x 1.5 = 7Kts.

 

[Alicatt]

You mean one of these?

 

[alexhibbert]

There is nothing complex about calculating hull speed.
In a displacement hull it is approximately 1.5 times square root of waterline length in feet. Adding more power simply has no effect unless there is sufficient to force the hull into a semi-displacement mode or onto the plane.
Assuming your waterline is 7m, ie 22 ft, the hull speed is 4.7 x 1.5 = 7Kts.

I'm familiar with the Froude calculation, but am under the understanding it's a useful basis, but then modified by a load of other factors. But yes, the advice that 8-10kts is possible does appear false.

 

[Bajansailor]

Alex, are you planning on converting your lifeboat into a cruising yacht?

If so, there is a long thread on this subject on the Boat Design Forum here -
What could be done with an enclosed lifeboat?

 

[alexhibbert]

You'll reach your hull speed, which is circa 6 knots, very efficiently. After that, you may be able to eek out another knot or so, but that extra know will probably double your fuel consumption. Cannot imagine ever getting to 10 knots in a boat of the size and style I'm imagining from your description, particularly with only 48hp. Maybe with a few hundred HP you might get near it, but fuel will be multiple times what you burn at 6 knots.

That's certainly what I found on an initial test. Around six knots is easy low down in revs - and then tons of wash and noise to get another knot or so.

 

[alexhibbert]

Alex, are you planning on converting your lifeboat into a cruising yacht?

If so, there is a long thread on this subject on the Boat Design Forum here -
What could be done with an enclosed lifeboat?

Thanks for the link. I'm a fair way along the conversion, and yep have seen some of the efforts on other TELBs, eg. Stodig.

 

[Gary Fox]

I was going to mention Stødig..
I don't think you'll be able to make her go any faster than hull speed to be fair, and as a cruising yacht I guess she will be lightly loaded compared to her intended task. Is it the original engine? Excess HP would help to shove her through a swell full of survivors, but doesn't convert to more knots.
PS, piccies please

 

[alexhibbert]

I was going to mention Stødig..
I don't think you'll be able to make her go any faster than hull speed to be fair, and as a cruising yacht I guess she will be lightly loaded compared to her intended task. Is it the original engine? Excess HP would help to shove her through a swell full of survivors, but doesn't convert to more knots.
PS, piccies please

There are some videos here if you're interested: https://www.youtube.com/channel/UCBk2kqWm36UsD2HxlMd3q4Q

Yep, understood that speed will be limited. It's no major drama as many small yachts only cruise at that sort of pace. I'm more interested in the boat's ability to fight an adverse wind or current. Yes it's the original 2007 Bukh.

The boat is rated for over 10,000kg, and stripped out is around 2,000kg. With equipment and a hell of a lot of fuel, I'm estimating no more than 6,000kg. It leaves room to add steel ballast to the best spots to improve sea handling.

 

[Tranona]

The test of whether the prop is OK is whether you can achieve close to full rated RPM in flat water and a speed in excess of hull speed. This should then allow you to cruise at around 90% hull speed at 75% rated power. sounds like you are doing this (subject to the revs bit). One of your problems is the wide range of displacements you quotes. 48hp is more than enough for 6 tonnes, but marginal for 10. Another problem as you have discovered is the lack of buoyancy at the stern which digs down once you exceed hull speed.

I would expect you to be able to cruise comfortably at 5-5.5 knots in most conditions you would want to be out in.

 

[alexhibbert]

The test of whether the prop is OK is whether you can achieve close to full rated RPM in flat water and a speed in excess of hull speed. This should then allow you to cruise at around 90% hull speed at 75% rated power. sounds like you are doing this (subject to the revs bit). One of your problems is the wide range of displacements you quotes. 48hp is more than enough for 6 tonnes, but marginal for 10. Another problem as you have discovered is the lack of buoyancy at the stern which digs down once you exceed hull speed.

I would expect you to be able to cruise comfortably at 5-5.5 knots in most conditions you would want to be out in.

Thanks for this - sounds spot on. There's worse than 5-5.5 knots!

 

[Robin]

There is nothing complex about calculating hull speed.
In a displacement hull it is approximately 1.5 times square root of waterline length in feet. Adding more power simply has no effect unless there is sufficient to force the hull into a semi-displacement mode or onto the plane.
Assuming your waterline is 7m, ie 22 ft, the hull speed is 4.7 x 1.5 = 7Kts.

usual figure is 1.34 x squ root of waterline length.

 

[Slowboat35]

I don't know about you but I find multiplyng by 1.5 in my head rather easier than by 1.34, and the difference so slight it hardly matters.
Hence the use of the word "approximately" in describing a rule of thumb where two decimal points of accuracy is frankly pointless.

 

[oldmanofthehills]

usual figure is 1.34 x squ root of waterline length.

I was always taught sqrt (2 times water line length in feet) ie 1.414 times sqrt 25 in this case. The older wooden boats of earlier years had about 1.25 x sqrt wll. A lifeboat might be kind of chunky and tend toward the lower end, certainly my "lifeboat hull like" LM27 is not very slippery and thus rather sedate.

A planing hull avoids the effect by getting so much out of the water that the drag which occurs at displacement speed limit, is not enough to stop the boat accelerating further.

 

[TLouth7]

Yep, understood that speed will be limited. It's no major drama as many small yachts only cruise at that sort of pace. I'm more interested in the boat's ability to fight an adverse wind or current.

The good news is that it sounds like you have lots of reserve power to push you at the same 6 knots even into plenty of wind. On the other hand I guess you will have a pretty significant amount of windage.