Increasing the planing surface with a swim deck

Questions about modifying a design

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Signor72
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Joined: Wed Aug 03, 2016 1:20 am
Location: Newcastle, NSW, Australia

Increasing the planing surface with a swim deck

Postby Signor72 » Sat Aug 06, 2016 2:44 am

I'm thinking hard here!

If I put a swim deck on a barrel back (for example) that was at the same plane as the hull - this would extend the planing surface of the hull (?/!)

The boat wouldn't be too affected at rest (?/!) but would likely be a little more stern light once the boat was on the plane (?/!) and may actually use less fuel as the attitude of the boat would be slightly more parallel with the water surface(?/!)

Would this then require/allow for a slightly further aft balance point in the vessel overall (?/!)

Would it be fair to say that the end of the swim platform would constitute the stern of the boat in terms of the water 'footprint' of the boat?

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kens
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Location: Coastal Georgia

Re: Increasing the planing surface with a swim deck

Postby kens » Sat Aug 06, 2016 9:23 am

You're thinking is nearly correct.
Yes it would act like fixed trim tab, however, with a straight inboard like a barrel back, the inboards already run good as is.
With engine weight already in the center of the boat, why do you need that?
Do you have an inboard powered boat now?
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

Ranny
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Joined: Wed Jun 25, 2014 3:01 pm
Location: Lake Monticello, VA

Re: Increasing the planing surface with a swim deck

Postby Ranny » Sat Aug 06, 2016 2:55 pm

I used to have an 18' Duffy Electric Catamaran. I believe the LWL Waterline Length was about 16'. If I walked forward, two things happened. First, my wife would complain that no one was at the helm. However, we were only going 5 MPH. Second, the speed would improve at least 1/2 MPH.

So, the answer appears that your displacement speed should increase by 1.34 X SQRT of the increase in LWL. That is also why old time sail craft were really long.
Ranny

Signor72
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Joined: Wed Aug 03, 2016 1:20 am
Location: Newcastle, NSW, Australia

Re: Increasing the planing surface with a swim deck

Postby Signor72 » Sat Aug 06, 2016 7:53 pm

No I'm planning a boat based on the various barrel back designs and am hoping to arrange the seating further back in the hull - all just ideas at this stage.

garrech
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Location: South Carolina

Re: Increasing the planing surface with a swim deck

Postby garrech » Mon Aug 08, 2016 4:53 pm

I could be missing something here, but all the swim platforms I've seen only contact the water when the boat is sitting still or at idle speed. Once you accelerate, the water is still being displaced (pushed down) by the hull and doesn't get back to lake level until a few feet after the platform. In other words, at speed there is an air pocket between the top of the water and the bottom of the swim platform. Does that make sense?

Signor72
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Joined: Wed Aug 03, 2016 1:20 am
Location: Newcastle, NSW, Australia

Re: Increasing the planing surface with a swim deck

Postby Signor72 » Sat Aug 13, 2016 4:34 am

Yes it makes sense.

My hypothetical suggestion was about swim platform that constructed in the same plane as the hull so as to effectively lengthen the planing surface when at planing speed.

It was just a thought...

Kevin Morin
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Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: Increasing the planing surface with a swim deck

Postby Kevin Morin » Sat Aug 20, 2016 12:16 pm

Signor72,
once a boat is up on step; "planing" the primary factor in top end speed is HP/Wt. ratio or power on the prop compared to the all-up displacement of the boat. That means that increasing horsepower for the same boat will speed her up more than changing other factors except reducing her displacement.

With that said, (HP/WT ratio is PRimAry influence) Length to Beam Ratio is also a big factor in speed/performance IF the HP and Displacement are left fixed. So a 5' wide chine-to-chine X 15' running waterline boat with 100hp @ 1.25tonnes (2,600-2,700lb.) will travel faster than the same boat that is the same bottom width but only 12' long running waterline.

Longer shapes, for a given chine width are more 'slender' in comparison so the Beam (fixed in this example) remains the same and the planing area becomes longer ( length) therefore the 12' X 5 is a 2.5 ratio versus 15’ x 5 or 3.0 ratio means there's a greater ‘slenderness’ in the longer shape’s proportions. The longer slender shape has less resistance or drag – leaving displacement and power the same, the reduction in resistance will become increase in speed.

There are couple factors involved- one other factor in bottom design is the waterplane loading or how much the bottom has to carry per square area where the longer bottom is greater area so a smaller load per area, and the other is the shape proportion or beam-to-length (length-to-beam inverted) ratio.

But these effects have their influence on the boats’ performance in relation to the speed ranges too!! That really complicates things for the designer. Dividing the speed by the Length gives another ratio for hull comparison. At speeds-length ratios below 1.5 or displacement speeds the effects of the other ratios shift! (you didn’t ask a completely simple question…) Then a couple other ranges of speed from1.5 to about 2.5 ratio of S/L ratio or semi-planing (almost sort of planing) the design factors behave somewhat differently as well; and from 2.5 S/L up to say 3.7 or 4.2 ratio (above that is high speed racing and not being discussed here) a further set of responses and performance will result if the Length to Beam ratios are shifted.

So, attempting to address your question about adding a planing bottom hull extension the boat will travel faster for the same thrust with that extension but it may (as Kens has noted) also change the running pitch (bow up or down) depending on how she’s balanced with the shorted planing bottom. Next, if the swim deck/platform is boxed in to the at rest waterline is will add buoyancy to the stern and she’ll sit just a bit higher at the stern compared to the same hull without- IF… the engine & fuel are left in the same (shorter) hull location. If the displacement moments are all drawn from scratch based on the true underwater volumes, then the engine and fuel would shift to balance the boat just slightly aft.

Yes the aft end of the planing surface would be the aft most point of the waterplane in either running or at rest attitudes. Since the bottom will be submerged to some depth while at rest, adding some short sides/"boxing in" would add at rest buoyancy to effect the other points in your post. Fuel use can be inferred from the discussion above about resistance- instead of considering top end, adjust the discussion for fuel use (still related to drag) where a fixed throttle or power setting uses less fuel to achieve the speed desired due to reduced shape drag. How much savings? No prediction here; "Some". Is some a ten dollar bill an hour or twenty cents? Can't say.

Hope this helps add a few cents to your thoughts as you plan your boat? I’d recommend looking up some of the of the many good books on boat design; Dave Gerr’s “The Nature of Boats” is one that is helpful. Another helpful idea may be to go through the catalog of designs here, and make a table (spread sheet) of the various designs, in various LOA's and list their chine and waterplane lengths. Add power, do some research on performance and you'd have some reliable feedback about your questions. The table of design info could be extended to others' designs as well, then you'd have real world figures to reference in your design cycle.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin


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