View Full Version : hull speed round vs. flat sterns
12-12-2001, 08:00 AM
I have a couple of questions concerning transoms on small planing hulls. All other things being equal on a given planing hull ( say in the 16 foot range ) would a flat raked stern or a round stern ( such as a Hooper Island skiff ) be a faster boat given the same length.
Similar question about out board powered planing boats with motor wells-would a boat with the out board hung on a flat raked transom be faster than the same boat with the motor hung in a motor well say 18 inches forward of the transom?
These questions have had me pondering for some time and I am interested in the answers that are offered.
12-12-2001, 10:42 AM
Your first question has no easy answer. Assuming the same bottom shape, the speed of each type will depend on other factors such as weight and buoyancy distribution.
If a boat is properly designed as either a transom mounted or a well mounted engine, it will probably not do as well if the engine is simply moved to the other location. The well area would, of course be closed or opened as needed for the move.
However, my small shrimpboat with a 50hp outboard mounted inboard, was slower and less maneuverable than its stern mounted cousins. This experence has been verified by others in my area.
I'm not familiar with the Hooper Island Skiff, but but I am guessing that by "round stern" you mean transversely (side to side) & that there is still a clean break between the bottom & transom. If you are talking about strickly 16' from extreme to extreme, you may be a bit better off with the flat transom, though it may be a bit slower if we are talking about high speed. The flat transom will aford you a bit more surface area of bottom at the transom end, where it is needed to 'get you up on top'. Likewise, the added wetted surface will translate to added drag at very high speeds once you are up & going.
A class of boat that I grew up with is the "Jersey Speed Skiff", which is a 16', basically flat bottom, fast(read that as racing), inboard. These boats are limited to 16' overall, and in the early days, to help performance, the bottom planking was run out aft of the transom to form what was called a "squat board". Some builders would cut the plank ends in one big semicircle, & some in a point (triangle) shape, so that the planking actually ended anywhere from 8" to about 14" behind the transom, depending upon builder. The specific shape didn't really seem to affect performance.
As to transom mount vs. in a well. The only high speed problems I can see with a well would be possible uncontrolled spray at very high speeds.
I guess I should have started out by asking what do you mean by "high speed". Do you mean 30 knots or 60 knots?
There are a lot of issues (advantages/disadvantages)other than just top end speed to consider in your question. How would you be using this boat & what are your expectations & then I can give more insight.
12-13-2001, 08:45 AM
nedl- I guess what I mean by high speed is about 30 kts-That is high speeed to me when on the water. The boat I am building is going to be used mainly for traveling to and from local barrier islands traveling the intercoastal and protected waters.- I want to use the current motor I have which is an old 20 hp Mariner. The boat doesnt have to be off shore able, but I want a fairly sea worthy boat and to get the most speed possible out of the motor I have got on a 16 foot boat. I would prefer a motor well, but wonder if I would be sacrificing speed if I hang the motor in a well instead of on the transom. I have always liked the classic look of a rounded stern as well with the clean break between transom and bottom..
12-13-2001, 09:22 AM
Just up the coast from you there is a boat very much like you are talking about. It was designed by Graham Byrnes of Vandemere, NC and makes its home on Orchard Creek off the Neuse river. "Miss Stephie" has both a round stern and an engine in a well. It has a 15hp and makes a bit over 15kts but would accept your 20hp.
Here is a link to a picture.
[This message has been edited by Tom Lathrop (edited 12-13-2001).]
The folks at Glen-L have a couple of dory plans I have been considering. These have a well but I think they include an option for transom mount. They may be able to answer your question on the gains/losses of each approach.
At 30 kts. I don't believe you will see any difference in top end speed based solely on transom shape, so I would say go with what you like. I myself have always liked outboard wells (though they do "eat" up some interior space) and like seeing the true transom that they allow. You are also less likely to be pooped if caught in a following sea; this is also because you have moved all that weight about 24" farther forward.
12-13-2001, 06:25 PM
Thanks to all for the replys to my questions- they have been very helpful- I have seen the Graham Burnes design Miss Stepphie- saw it jon an old post- I actually started to build the 17 foot Bay River Runner, of Burnes design, however once I got a few of the pieces cut out I decided it was just too large a hull for what I wanted. I likeed the shape though so I shrunk it down to a bit less than 16 feet ter with a good bit less beam-. I still havent decided what type of stern/transom I want on her Thanks to all once again.
[This message has been edited by liveoak (edited 12-13-2001).]
12-13-2001, 07:31 PM
When you shrunk the 17ft Runner, you were getting very close to the 15ft Bay River Skiff/Runner. The difference between the Skiff and Runner being only that the runner has slightly flatter aft lines for higher speed. I built the BRS15 and loved it under power or sail. A far better boat than WoodenBoat's "Perfect Skiff". Sold it to get the cash to build another larger boat.
It's all personal preference, but I think the square transom with about 11 degrees rake is better looking than either the round or ducktail types.
12-14-2001, 08:11 PM
Tom- I know well about the 15 foot BRS/BRR - I worked on a couple of them and built a small center console for one while working for Nelson Silva. Those boats have an incredible ride and I feel it would prove difficlult to find a more " perfect skiff "
[This message has been edited by liveoak (edited 12-14-2001).]
12-15-2001, 11:56 AM
It was a sad day for wooden boat lovers in NC when cancer took Nelson Silva. He had said that he preferred the Bay River boats to the Simmons Sea Skiff.
On a Sunday afternoon a few years ago, Liz and I were sailing "Loon", our BRS 15 in Penobscot Bay off Castine, ME. A beautiful downeast schooner sailed by and the crew called out, "that Loon is a beautiful little boat". We returned the compliment with pride and sailed on.
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