keel trunk

[rapscallion]

http://www.sailinganarchy.com/forums...=post&id=38233



I was wondering if the keel trunk would adversely affect the performance of the boat...

could this be done on a 25 to 28 foot boat without adversely affecting performance?

[Raggi_Thor]

I don't think it affects performance noticeably, but it would be better if the step was smaller.

[Tim B]

It will adversely affect the performance, there'll be a big vortex off it when sailing, and that means unnecessary drag. Presumably it's been done to get more depth inside the yacht to raise/lower the keel. Normally the keel trunk would stop flush with the bottom of the boat.

One can only assume that the designer had a good reason to do it the way he has.

Tim B.

[rapscallion]

If a boat builder made the "step" between the trunk and the keel less obvious could a design like this work? When I say work I mean it wouldn't adversely affect performance significantly (6 to 9 seconds a mile)

[Raggi_Thor]

I am no expert, but I don't think the extra vortex will be big.
You should of course make the step as smooth as possible.

But why do you need the extra trunk below the bottom?

[Man Overboard]

I agree with Tim, I see no reason for having a step. It adds virtually no additional strength. (at least to the structure). It limits how high the keel can be lifted, which kind of defeats the purpose of a lifting keel in the first place. It must be a fairly deep keel; it doesn’t look as though it is sized properly for the size of the boat. It could be that it is to help support the weight of the bulb while heeled, maybe to keep the keel from flexing so much.

[Raggi_Thor]

Look at the picture.
The rudder seem to be of constant draft and the deepest point of the boat.
There is a saildrive to protect.
I think they want to be sure the bulb is lower than the propeller.

[Man Overboard]

Yes, good point about the sail drive, but why design in this fashion? It doesn't seem very thought out as a whole. by moving the sail drive back a foot or so, the bulb could retract all the way to the belly, and still protect the sail drive. But why such a deep non retracting rudder? In any case, the trunk doesn't need to protrude to protect the sail drive unless you expect the keel to fall off. (kidding of course).

[Chris Ostlind]

Protect the saildrive or not, that's a huge flow disturbance device, plain and simple. As if the saildrive weren't enough a turbulence generator..

If they were going for a slide stop for the retractable keel, all they needed to do was make it an internal lift stop setting beyond which the lifting system could not go without an override.

Besides, that form could have been faired a whole lot better into the shape of the strut.

An unusual solution, IMHO.

[Raggi_Thor]

I agree it's not an ideal solution :-)
But do you really think that step create a lot of drag?
At that position of the keel you will have pretty parallell flow?

Forgot, maybe they want to dry out and rest on the keel :-)

[Man Overboard]

But do you really think that step create a lot of drag?

I guess that is a good question. I think it is safe to say there would be increased drag, but to quantify that would be a guess. It is safe I think to simply conclude that it is unnecessary.

[Tim B]

The flow is not going to be parallel to the step when you get heel pitch and leeway involved, all of which will be present. It's a bit like reducing the aspect ratio of the keel towards that of a wing of appropriate span. It will probably negate most of the increase in AR that the proximity to the hull gives, which means greater induced drag. In addition there will be greater skin frictin drag too.

This is not a good solution.

Tim B.

[rapscallion]

If there was a minimum depth the keel had to retract to, because of the customers request, I was thinking that placing the trunk under the hull would be a way of reaching the minimum depth without having the trunk take up too much space in the interior of the boat. If the reduced performance is (minimal -- subjective) maybe this type of option would be viable. I have no idea how to quantify the performance of the keel, open foam perhaps?

[custom30]

I am currently in the start build stage of my boat. A very popular 30 foot racing boat ( MORC PHRF etc. ) I would like to make it a lift keel ( melges ? ) for ease of trailering . Does anyone have any ideas on how to do this effectively ? please email me for more info.

[Tim B]

There are a large number of lifting keel boats that look very nice indeed. Notably the keel should look (externally) no different to that of a normal fin-keeled yacht (except the it is constant-chord and constant-thickness, of course) Take care with the ballast position, getting everything where you want it is not easy.

Yes, OpenFOAM will do an analysis, but it is not the simplest thing to do, unless you have the time and the right bits of kit. Time is an ever-precious commodity that I never have enough of. I could do the analysis, but it would take a while! (even on the faster solver I have for immersed bodies). The question is why bother? the way to reduce the drag from it is to get rid of the damn thing.

If it is to prevent lifting above a certain draught, a brass pin through the case at the correct height would be a better solution.

Also, saildrives will (inefficiently) produce some sideforce, which may be considerable on a smaller yacht. They are certainly preferable to shaft drives in that respect.

Cheers,

Tim B.