Warped plane vs constant deadrise

Hello all,

I have been away for over 6 weeks cruising in the Bluejacket 24 "Liz" in Ontario, Canada's Trent Severn Waterway and the North Channel of Georgian Bay. Sorry to have missed out on this discussion since it is very interesting to me. I learned a bit more about the boat from the trip also.

In getting back to the original subject of monohedron versus warped plane hull efficiency, I am firmly placed in both camps. My experience with my own boat and in towing tests show some advantages to both types in different conditions.

Briefly, I believe that the warped plane bottom offers less resistance in achieving planing mode and that the monohedron bottom is both better and more stable at higher speeds.

Since we are not talking about deep V hulls here, the monohedron hull has constant deadrise only over the aft planing sections and the deadrise in the forward sections is variable, rising to a fairly sharp entry forefoot.

In the model used for towing tests, only the aft bottom sections were were varied from constant deadrise to a warped shape by raising the keel toward the stern while keeping the chines parallel to the waterlines. At low speed, the warped bottom gave a slightly lower towing resistance. At high speed, where the monohedron was always stable, the warped bottom exhibited a tendency toward yaw instability. This got very bad when the speed was increased further and eventually resulted in foundering of the model and destruction of the towing rig.

Whether this was due to bow steering, stern steering impulses, both, or something else, I can't say, but I have some thoughts on it. I do know that many of the Carolina Sportfisher boats that Tad referred to have a bit of rocker in the aft bottom in addition to the warped bottom. When I asked a builder abour this, he said it was to keep the bow higher at sea. So it might be that they have one factor, warp, holding the bow down and another opposing factor, rocker, holding the bow up. I don't think this is the best way to do a design, but the local fishemen are happy with it. I do suspect that these are macho fishermen who want to drive a macho boat for macho customers and that the designs are evolved rather than being designed from scratch.

What speeds are involved? Perhaps up to 25 or 30mph, the warped hull has the advantage and the monohedron is better above that. Other factors are also involved, so this is just a gross guess. I was working with low bottom loading and an aspect ratio of about 0.34 with a CG about 37% of LWL forward of the transom.

 

Ahh - now we're getting somewhere: so as a general rule of thumb, one could say that the warped plane is more efficient at planing speeds to approx 25 mph, the monohedron is the way to go at speeds over 30.
I, of course, am chasing the impossible - a hullform capable of maintaining efficient planing down to around 12 - 15 knots, yet capable of cruising @ 30 + in surface chop!

Well actually, I was thinking of deep V's too when I posed the original question. Where, for instance would you place the Hunt 33 ( www.crhunt.com )? At 10000 lbs it is relatively light and with a single 370 hp diesel manages around 2 nmpg @ 20 knots. The boat can manage 32 knots flat out and I can see that if you wanted to charge around at that speed regardless of conditions, then the deep V hullform might make sense. But to my way of thinking - and based on your comments, Tom - you would have to assume that the boat would be better with a warped plane hullform......

The trip sounds terrific Tom - very jealous! I'd be interested to know what you "learned about the boat from the trip" too.....

 

____________________________________________________

I learned that "Liz" is very satisfactory for two people to make extensive cruises in comfort.

Trailering makes it possible to cruise in remote areas in limited time without having to tread "old water". We drove almost 3800 miles and cruised about 350 miles in the boat.

The Trent Severn Waterway and the North Channel are both excellent places to enjoy a cruise in a small boat.

"Liz's" top speed dropped about 3mph from the normal 23-24mph when lightly loaded near home to about 20-21mph with full cruising load aboard. Since I have had more weight than the cruising load aboard before and not had this much drop in speed, I attribute most of the loss to the added load forward. This increases the wetted surface and is most likely responsible for the speed drop. The good news is that she handled chop even better than before with the waves striking the higher deadrise sections forward first. Were these results also affected by the deeper immersion in freshwater?

A sun shade over the cockpit would be a plus and I'm looking at that possibility.

Fuel mileage for the cruise was 6.8mpg, which is about 1 1/2mpg less than we usually get on day runs. Average speed for the trip was 10.1 mph, including the locks and many no-wake areas on the Trent Severn.

Such cruises cost a lot of money but MUCH less than the larger boat people spend.

At the risk of boring you all too much, that's all for now.

 

Never happened yet!

Almost 7mpg! - That's almost as good as most American cars isn't it!!! It just goes to show what can be accomplished when a little thought - rather than a lot of unnecessary junk - is put into a design....

 

http://cnczone.com/forumdisplay.php?s=&forumid=5

 

Hi Guys:

I've done some model testing on this too and it was clearly demonstrated that warped planes are more efficient at lower speeds and monohedron or constant deadrise forms are more efficient at higher speeds. The question then, of course, is what's "lower" and what's "higher" in terms of speed? Our tests were not terribly comprehensive, but we did enough of them that I feel comfortable having placed the dividing line in my own subsequent work at a Volume Froude Number of about 3.3 (or a V√L of around 5.0, if you prefer). Faster is monohedron/constant deadrise territory; slower is for warped bottoms. In general, the slower you go, the more warp you want too, until water is no longer breaking free of your transom (somewhere around a Froude Number of 1.0), at which point you're going too slow to even have chines.

That all said, I would most heartily second the comments of others to the effect that there's a lot more to achieving a good overall result than picking the bottom shape. Most efficient isn't necessarily best.

 

I had intended to do more testing of the model for but one reason or another have been tied down with other stuff. I have looked into the Carolina Sportfisherman design a bit more. The reason for the aft rocker seems to be that they want to make more speed safely downwind. It keeps the sharp entry, which they like for going into waves, from digging into the wave ahead and causing broaches or at least discomfort (terror) to the customers.

I would think that the crossover speed between warped and monohedron hulls would be affected by length, bottom loading and aspect ratio. Length might be a factor because in a short boat the warping would require more curvature in the buttock lines and therefore more vortex to the wake.

Looking closely at the lines presented earlier by Dim, I think that what has been interpreted as hook in the hull bottom may not be that at all. It looks to me like the aft buttock lines are straight and the "hook" is the forward keel sections being lowered. This will keep the bow more in contact with the water at speed and make for easier wave entry. Such a design was used by Weston Farmer in a little boat called "Trumpet" many years ago. Trumpet also appeared to have a hooked bottom but closer inspection showed that this was not so. A possible downside may be a tendency to bow steer, especially when running at an angle to the waves. Or maybe not.

 

I have a ski boat with exactly the features that you describe Tom. She's rather beamy for her length and is pretty shallow on the deadrise front - it is a competition ski boat after all! - but the deepening of the forefoot for around the fwd 3rd of the boat improves it's performance in many respects. It doesn't pound quite as much when running up-slop as you'd expect (it's no offshore racer either...), it also truns like no other outboard powered boat I've driven, and it tracks better than many inboard, skeg equipped, boats that I've driven too. All in all it's a very impressive package - I'm yet to discover a handling vice of any kind

 

Hi Will,

That checks out. Would you say that the deep bow acts like a skid fin and that the boat rotates around it in a turn? What is the aft deadrise?

 

yep - the boat handles a lot like a good inboard ski boat, which are usuakky eqipped with 1 or more skegs located under the ski pole. It turns like it's on rails - at any speed!
aft deadrise is probably somewhere between 10 and fifteen degrees...

 

A bit of digression?

While not a professional (or even semi-professional) designer, I've "been around boats" (having been raised in the Puget Sound country of Washington) most of my life. I've done my share of boatwright work (some I even got paid for).

For the past thirty-plus years I've also had an interest, and participated in "hobby steamboating". This, of course, brings to many people's mind the classic fantail launch (ala Weston Farmer's Diana to name but one) with a slow (300rpm) turning, open engine and a large boiler, with a S/L ratio of 1. Or, if one thinks of high rpm, one thinks of the "buzz" of a 5000rpm hydroplane model.

Somewhere between these two is an area which appeals to me (and a lot of other hobby steamboaters). I have access to a company (well, I just designed a new website for them) which has designs and castings for a series of single acting, high speed engines in the up to 2000 rpm and up to 150HP ("TRUE" horsepower) range. However, I am a "tradionalist" in small (under 24' LOA) appearance and don't want to create a "look-a-like" fiberglass hull with a lot of flam at the bow and milk carton design (no offense intended to any participants in this thread as it's obvious y'all aren't into that either). Most of us would like a boat that can be "run up on the beach" without damage (you know, like what we used to do with dad's 18' outboard hull - going at 20mph and tilting up the outboard at the last minute. Gee, Dad, I don't know how that big ding got in the propeller?). Well, I'm certainly not thinking of anything that'll do that speed, but 10 - 12mph would be nice. And an easy way to allow for beaching is to not have a fixed shaft and prop coming out of a skeg. I'm planning to use a Mercruiser Alpha 1 sterndrive. With something in the 1.5 reduction ratio range and a 16" dia prop, there are a number of pitch sizes that will work nicely. So with this sterndrive and the engine I have in mind there'll be about 275 lbs. in the transom area. The boiler - as it is only 16"W x 24"L x 30"H and 300lbs - can be placed just about anywhere. This seems to lead one to the "double wedge" type design. So...

There are a number of designs I've looked at. Bill Atkin's Sea Bright skiffs have some good ideas and I've sent for study plans of his Sea Hawk (LOA 21' LWL 19'6" Beam 6'8+") http://www.atkinboatplans.com/. She'll drive up on a beach nicely and stay upright when the tide goes out. But I don't think she has enough buoyancy at the stern to support the sterndrive and engine. I've also looked at outboard motor driven "Santoise" design used by Caribbean fisherman around Guadelope. It appears to be very similar to the lobster boats used in Nova Scotia (figures - both came from Breton and Normandy fishermen). I am well aware that one should be very cautious in taking a "snip from here, a snip from there" in hull design, but there are features in each which will lead me to what I want.

Hard chine is also not the most desireable. I plan on using lapstrake, plywood planking using Ian Ougthred et al techniques. Any thoughts, suggestions, pointers will be welcome

Maybe this should be a new thread. If so feel free to move it.

Ron Fossum

 

I think I have solved all the hull design problems. Buy 1 boat of each type. I am beginning to beleive that all types of stern drives will always be "squirrely" because they have all their weight at one end. The boat has to "blow over- lift and dive stern first" . Simple laws of physics- throw one of those folded paper delta wings we made in school hard, they always lifted stalled, and fell tail, stern, first. Our boats will always try to flip once we get the bow up. All are balancing the boat beyond it's natural nose stays down during water or wind caused lifts. The old Hackers, Garwoods of that time period would lift their bow slightly when going into a wave taller then them. The cleaver ( vertical ) bows punch straight straight thru with little or no hull shock or passenger lurch at 40 mph. Wind or blow over is non existant because CG and MAX. speed CG are almost the same place. Thats why they are so stable when repowered with 800 hp . Ask a Geezer owner- simple big 3 blade and he's out there racing in a small circular course, no sweat.The HP EFF. of those boats is staggering compared to our sterndrives wasting HP to artifically lift the bow. Their only problem was rudders that were 6 sizes too small. Fix that and they will compress your spine into the seat. If speed is more important than your life- do it in a stern drive- you will always blow away any other type of present day drive.