Hull Types and Lateral Stability

[mackid068]

So, I've got a few minor questions.

1. On a semi-displacement powerboat, is a full keel a good device for lateral stability?

2. For a semi-disp hull, is it possible to go over 15 kts (if the boat is under 32')?

[woodboat]

My 38 owens granada was a semi-displacement with two 327 Chevys. It took WOT to get over the hump and on plane. Once there, speed came up pretty well and it felt as if it was going faster than 15 Kts. As soon as I closed the secondary and pulled the motors back to 3300 the stern would squat and it settled back in the water. So to answer number 2 I will say yes with a caveat. It depends on the particular hull and available power.

[mackid068]

Alright, sounds good. So, is a nice full keel good for stability on a semi-disp boat?

[Willallison]

more power... more speed.... of course it's possible to push a semi-displacement hull at over 15 knots - just take a look at some of the 'lobsterboats' that they race in Maine - up around 50 knots! The real question is whether you really want to: there comes a point where the installed power required to push a semi-displacement hullform at higher speeds becomes uneconomical. You'd be better off with a simple planing hull. Having said that, if you are planning on doing much running around at semi-displacement speeds, the a planing hull ( unless it is very light and of correct hullshape ) will handle like a dog.
Without doing the numbers, I'd say that as long as you're top speed is no more than about 18 knots, then thie semi-displacement shape can make sense - particularly if you often run at 10 - 14 knots.

As far as the keel goes, yes it's good for directional stability - once again, so long as you don't go too fast, and so long as you have a big enough rudder.

[woodboat]

Well my thoughts are that a full keel will help keep it flopping from side to side at anchor and pointing straight under power. I don't know what other possible effects it may have, positive or negative. I assume it will increase wetted surface thus drag.

[woodboat]

Oh the sweet thing about my semi was that with very little power she would do 10 knts and get incredible fuel mileage. I had the carbs tuned to be VERY lean in the 2000-2500 rpm range.

[marshmat]

I don't much care for semidisp at all, thanks to the ones around here that run around throwing up massive wakes that toss us little guys all over the map. The wake from a boat at semidisp speeds is usually high and short, and carries a long way much like the wakes of the hated fast-ferries. As a runabout pilot, I'd much rather face the wake of a yacht on full plane than that of one running semidisp speeds. Granted, I've never piloted one of these beasts and so can't speak for the genre as a whole from that perspective.

So yes, it's possible to push semidisps fast. Just stay a few hundred metres away from the rest of us when you do.

[KCook]

Shamrock inboard fishing boats have a keel -

Pretty rare in boats this small. But this keel is mainly to protect the prop during a grounding. Would add to directionaly stability. Lateral stability (against rocking) would not be so effective.

Kelly Cook

[woodboat]

I would call semi displacement speeds slightly higher than displacement speeds. Say a 38 ft would be at hull speed @8 Knts, the Semi would be there @10. I say this because for my Owens it made very little wake @10 and used very little fuel. It would do 10 @2200 rpm. When the throttles were at 3300 Rpm she was only going about 12 Kts but was makeing a great big hole in the water like I had never seen before. Standing 6 Ft tall in the aft cabin @3300 rpm you could NOT see anything out of the rear windows except water, so she was digging a 6 ft hole at least. I have seen the wake roll like a beach wave right over top of a pier that was at least four ft above the water. My wake did flatten out once up on plane. So those Semis running around making a terrible wake either need to go faster or slower. In my opinion a semi is only viable if you trully plan to run at lower speeds and have an engine that is efficient at those speeds. I prefer to cruise on plane at higher speeds with my new boat although not much higher, GPS shows 14 Mph cruise @3200 and 20 Mph WOT @4100

[mackid068]

Yeah, I'm not a "speed" person. I'm a dinghy sailor but I'm not a speed-demon. I just would like to design a slow boat that's comfortable in a seaway.