Hull Length Extension

[SuperPiper]

I asked this question on a previous thread but would like to resurrect it here.

I have a 18-1/2 foot LOA pocket-cruiser with a 15-1/2 foot LWL. The transom is 4 - 6" above the water while loaded and at zero speed. However, as soon as the small craft starts to make way the transom will drag 2 - 3" below the water level. Shifting weight forward (as in a dinghy) will get the transom out of the water and get the speed up.

So here is the question:
If the hull lines were extended beyond the existing transom, would this increase the effective LWL? Or, would this just increase the wetted surface area?

I don't know if there is a name for this kind of hull/transom treatment but it seems to be popular with both dinghies and Open x0 racers. Does the stern wave peel away smoother from the scarfed hull than from the blunt hull?

[gonzo]

An extension would work like trim tabs in a powerboat.

[Andy]

I think there are a number of issues here. The main question is whether or not the displaced length of the hull would be increased due to the extension, or if it would just add mainly wetted surface area (and thus drag) on its own. More displaced length would mean more waterline and a higher displacement speed. Most boats designed to exploit this extension (mainly metre boats and Americas Cup class racers) are designed so that as they heel, the volumetric distribution changes slightly in such a way that the hull itself sinks slightly in order for the extension to make contact and be immersed sufficiently in the water. This has the bonus of low wetted surface when upright, and long length when heeled.

Therefore, for this mechanism to work effectively, it is easier if the hull is narrow. Without knowing the stats of your boat I would guess that if it is a typical 'fat' trailer sailor then it may be tough to get the sinkage you require without a weight redistribution (not recommended without careful thought!). On the other hand, how much of the transom immersion when moving is due to the stern wave system? If you think that either the hull is sinking when heeled, or that you really can displace the stern wave aft with an extension, then it would be worth a go. You may also be able to clean up the flow around the edge of the transom a bit, but if you look at AC boats with huge overhangs sailing, you can see that even they drag their transoms sometimes.

Do you have any photos of the hull of your boat at rest and sailing? These would help in the guessing work!

Andy

Andy:

You are right about the shape of this little boat. The hull has a moderate Vee at the entry but changes to a 3ft or 4ft wide flat bottom with pot-bellied bilges within about 1/3 of its LWL. The flat bottom exists for about 1/3 of its length and then the hull rolls up to a semi-circular transom.

According to the literature, there is 10" of rocker from the belly to the water line ahead of the transom. Therefore, the total rocker from the belly to the transom must be 14 - 15".

I suppose that extending the hull makes the beam:length ratio marginally narrower. However, adding the length at the stern only will effectively shift the maximum beam forward along the LWL(?).

Is there anything to be gained from extending the hull? The easy answer would be NO. But, we have seen the AC boats get a complete facelifts between rounds.

What do you recommend?

[SuperPiper]

While waiting for my daughter to finish work at Chapters bookstore, I thumbed through a magazine that I had not seen before: Blue Water Sailing.

There was an article by Bob Perry that compared the performance of 4 typical bows: standard, plumb, spoon & long. A table of VPPs in the front of the article indicated that the long bow had greater speed in all scenarios. Bob also made mention of the Davidson AC bow. I did not have time to finish the article.

I am now wondering if a nose-lift is the modification I need to consider.

[gonzo]

I think that article failed to mention they are comparing waterline lenght and not overall lenght.