Express Cruiser

[Willallison]

Tom,
I've posted the lines for the boat at

http://www.boatdesign.net/gallery/sh....php?photo=707

I took a look at the images that I posted before, and I think you may be right - they probably give a slightly distorted view of the boat. The section lines for instance, are straight, whereas in the other images they appear concave.
You will see that the lines run straight for about the last half of the boat. Fwd of this the fairbody sweeps up towards the bow before turning fairly abruptly, as you noted, to meet the stem.

I wanted to keep the forward sections as deep as possible in order to minimise slamming when running into a choppy sea - hence the quite sharp change from stem to keel. By sweeping the bow sections up as I have, I would expect them to run almost free of the water in callm conditions, but still allow the boat to 'slice' through a moderate chop without having to slow too much. A time will come where the boat becomes uncomfortable at high speed - and like you I like a boat which can still run at intermediate speeds without falling off the plane. Hence the flatter aft sections.
The fairbody's upsweep also serves to help keep the bow higher in a following sea - and in effect produces a sort of rocker in the bottom as you refer to in the Carolina boats.

Any further thoughts?

[Willallison]

Incidentally, @ 25% of lwl, deadrise is 37 deg

Interesting concept.However, your views of Surface piercing drives are inaccurate.I am the Canadian agent for Q-SPD surface drives and there are models available to suit most needs and most desired speeds.On a vessel of this size a trimmable unit is not necessary,a fixed unit either SD 100 or SD200 would work for you.If you wish to learn more about surface drives and their applications try www.qspd.com ,Q-SPD drives can be had in either 100% carbon fibre construction or a less costly composite and it should be noted that with this method of construction you(the end user) benefits from total drive weight of approx. 50%less than with other well known drives.Also there are no bonding/corrosion issues as with say an Arneson and an aluminum boat.

[Jeff]

You mean http://www.q-spd.com/

Welcome to the forums too!

P.S. If you take a second and register and login, you can edit your posts in case you want to correct any typos or edit after the fact (a very handy feature I've found for my own posts too

[Tom Lathrop]

Will,

The idea of "slicing" into waves sounds attractive and is fine for slow monohulls and all multihulls but, I think, is not what you should want for a planing boat. There several breeds of boats that do this but none are what I would consider pleasure boats. You need an entry that not only "slices" but gradually gives significant buoyancy so that the boat is lifted over the wave without excessive shock loads. I would argue that the sharp entry on your boat has much negative and very little positive virtue.

The point is that the bow will have far too much steering effect than advisable when running into waves, especially off angle. The center console fishing boat that I referred to would try to throw the pilot overboard with a sideways twitch when driving into quartering waves and it was not as deep or fine as yours.

[Tom Lathrop]

Will,

Just had a look at the second set of lines. The bow does not look as fine there as on the first set but I'll stick by my comments. Take a look at the entry half-angle on some boats that are considered good in the regime you want to operate. I think you will find them less acute.

Even if my criticisms are too severe, maybe they will make you look at the issue more closely and teach all of us.

[Willallison]

Guest (Q-spd agent):
Thanks for the response. I have in fact designed the transom around the use of the Q-spd unit. I like its simplicity and as you suggest there are other desirable aspects of the unit as well.
I have contacted Q-spd previously with a short description of the boat and my design objectives and they were good eneough to respond - just as you have - with the suggestion that surface drives are not incompatible with my objectives. (In fact they were the only ones who bothered to respond - another reason for me to favour their use). But, ever the sceptic, I like to throw my ideas onto this board for the review of others - usually with far more experience and expertise than I.
As Jeff suggests - take the time to register - boatdesign.net is an excellent site and I for one would certainly appreciate your input.


Now, onto those other experts I was referring to......

Tom,

The idea of "slicing" into waves sounds attractive and is fine for slow monohulls and all multihulls but, I think, is not what you should want for a planing boat. There several breeds of boats that do this but none are what I would consider pleasure boats

Why? If the alternative to slicing is pounding, then I think I prefer the former. I'm not talking about running the boat at 35 knots in a 4 ft sea. More like 20 - 25 through 1 - 2 ft chop. This boat would only remain comfortable as long it maintained contact with the water. As I've said before, one of the fundamental design objectives here is to be able to choose a comfortable speed to suit conditions.
Having said all that, I take your point on board and will do as you suggest by comparing to similar vessels (so far as they exist). The forefoot may indeed be too deep - the design is only preliminary (and theoretical) after all.

You need an entry that not only "slices" but gradually gives significant buoyancy so that the boat is lifted over the wave without excessive shock loads. I would argue that the sharp entry on your boat has much negative and very little positive virtue.

With the wide chines, and fairly significat flair, I thought I would have achieved this....How would I go about creating more buoyancy, short of incorporating convex topsides, whic would make for a very wet boat?

[Willallison]

Q-spd:

Another quick thought:

there are models available to suit most needs and most desired speeds

It's more the ability of the surface drive to run efficiently throughout the vessels speed range that concerns me. You only have to read the magazine reports on boats like Pershings etc to see that surface drives are far more effective than other drives when running fast. But here I have a boat meant to plane efficiently at anything from the mid teens up to 30 or 40 knots. Is the surface drive up to that?

[Tom Lathrop]

Will,

I think we are at odds in the meaning we give to the term "slicing". I take it to mean that the boat tends to go through waves instead of going over them. The sharp bow alone as you show it will not do that. There is not enough buoyancy in the bow to affect the lift of the boat one way or the other. The rest of the bottom will quickly add buoyancy and lift the boat over the waves. In that sense I think the sharp stem has no effect in teh boats attitude when encountering waves.

What remains is the steering effect of the sharp bow when it enters a wave. This is not desirable when going into waves at an off angle and can be dangerous when running downwind. It is only the knuckle of the stem that I am referring to. If the stem were given more rake and blended into the hull with appropriate streamlines, my objection would be eliminated. I think the attitude of the boat in waves would be relatively unaffected except that the undesirable steering effect of the bow would be greatly reduced.

While the action of the boat in chop is important, the effect of larger waves that the boat will certainly be caught out in must be considered also.

These are just my opinions from a quick look at the graphic. They are given with the best intentions and not meant to be critical just for the sake of criticism.

[Willallison]

They are given with the best intentions and not meant to be critical just for the sake of criticism.

Exactly how I take them too Tom - and similarly I'm not arguing 'cause I think you're wrong: I'm trying to learn, and you seem to be a most competent teacher!

As far as the "slicing" goes - as usual, you're right. I tried to fiddle about with the forefoot shape a little yesterday. Unfortunately, learning the art of CAD isn't one of my strong points and I seem to have used a rather complex means of producing the hull - one which I'm struggling to alter!

[Hydro]

just another angle you might find interesting regarding fine entry bows at this link.
http://www.navships.com/hullforms.html