Effect of rising buttocks on planing hull

Dear Gentlemen,

i am currently working on the design of a 22' center console boat as a self building project - specs below:

LOA: 6.76m
Beam: 2.4m
Displacement: 1,45to
Target cruising speed: 22-25kn
Outboard powered
Strip diagonal planking

In order to keep the transom just as far immersed as it is necessary, i decided to go for a variable deadrise - thus having buttocks with an angle of approx 2deg upwards, while the chines are angled down 1deg.

The concept for this kind of hull was inspired by the dutch "Brandaris Barkas" boats, which i just admire for their planing behavior (see video)

http://youtu.be/kbsraKu8OYE

However, i have just seen pictures of the hull - no linesplans, etc...

And in the literature i studied so far, there are partly contradictionary statements to this kind of warped hulls.

I therefore would be thanksful for feedback, whether the hull shown in the linesplan could show similar planing behaviors.

 

Presumably you are using protected waterways, the boat you illustrate would be a menace on the open sea, imo. It would not require much encouragement to broach, with the lack of any skeg. For the sort of speeds you desire, the warped plane is OK, but perhaps a little wet in a chop with the near-plumb bows.

 

Well... the boat will most of the time operate in coastal waters and protected waterways between the adriatic islands of Croatia. Skeg and stem will be added, but not drawn in freeship. Some more flare in the bow sections could be taken into consideration for a more dry ride.

The main issue i would like to point out is the character of this kind of hull in view of it's trim and planing behavior. The Brandaris Barkas mentioned before show only very little change of trim - even during the transition from displacement to planing speeds.

Similar characteristics i've seen on Tom Lathrops Bluejacket boats, but he's obviously using a different approach than the dutch designers.

As it is visible from the picture below, their hull is also warped with rising buttocks aft, a sharper forefoot and a deadrise of maybe 8-10 deg. or so on the transom. Unfortunately the chine angle is not so good detectable from the picture.

So what do you think... would these kind of hull contribute to less change of trim during transition?

 

Should come onto the plane "flatter", and be stiffer in pitch on plane. This kind of thing is more like a harbour launch than something to take on long stretches of choppy water, where the flattish underbody will be exposed at the cruise speed you mention, though you should be able to back off speed OK with that kind of hull.

 

There is more to judging how a boat will perform than just looking at the lines plan. Looking at the video shows a fairly heavy and powerful dayboat that runs with the bow immersed. That generally promotes good characteristics in chop with the relatively fine bow as shown cutting the water. It does not appear to be excessively wet in mild water but spray blowback will drench any aft passengers running into a quartering sea. Controlling spray in those conditions is almost impossible unless your outrun it.

Compare it to this one: https://www.youtube.com/watch?v=IgY_PS6b_hA This one will be much rougher riding in a chop with the flatter aft sections striking the water rather than the finer bow sections.

Its all about the trim angle in these two boats. You can't just decide on a trim angle, it must be designed in with bottom design, weight (bottom loading) and weight distribution. Whether your boat will run at the same attitude as the Brandaris boats is not known.

Minimizing a transition hump requires careful attention to many factors but it can be obtained from differing criteria depending on the designers overall intent. I suspect that a boat built to your listed specifications will perform very well but I don't think it will behave like the Brandaris boats. Brandaris appear to be very fancy and very expensive toys for the very rich as well as heavier than yours is likely to be.. With all the mechanical gadgetry and fine finishes, they still fall back on a toilet stuck between the berths...ughh.

Another 22' runabout with similar intent to yours is being designed and built by Denny Wolfe at:
http://forum.woodenboat.com/showthr...owerboat-abuildin/page3&highlight=denny wolfe

 

Thank you Gentlemen for your constructive comments.

I am quite happy to conclude that i can go ahead with the concept - with some more attention on the overall package (weight distrubution, design of forefoot, etc...).

Tom, thank you for the other video. Very interesting to see the difference between those boats, indeed. With that kind of trim and a more or less flat forefoot, i can imagine that riding a chop would be less fun.

Another point worth to mention here is that many of the Brandaris boats seem to be equipped with interceptors, which certainly contribute a portion to the immersed bow while planing.

Nevertheless, i just really like the idea and the style behind. And in terms of different sea conditions, i guess that some adjustment with the outboards power trim will help to keep the boat safe and avoid bow steering in choppy waters.

By the way Tom... as soon as a closed cabin boat will be required, the Bluejacket will be on top of the list ;-)

 

Yes, interceptors on the transom will control trim angle and may explain the running trim in the videos. I wondered about all that weight of two engines in the stern not giving a higher bow attitude. I don't see the Brandaris bow sections as having any problem with bow steering as it appears to be a good compromise between fineness for smooth riding and fulness for good buoyancy.

Whether outboard tilt can significantly change the running trim of a boat depends on the natural longitudinal stability of the boat. On some boats outboard tilt is very effective and on some others, not so much. With interceptors deployed, outboard tilt will likely not be very effective.