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Old 09-17-2015, 10:50 PM   #1
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Boat design guy input needed.

Ok, I don't understand the physics of how my boat operates. I have a 42' boat with a 38' waterline. I think its 30000 lbs. We have twin 210 HP Cummins with 25X27 props. I can run 8kts at 1400 RPM. The boat rides flat at that speed. Any more than that and the stern starts to squat and the bow come up. I do 9 kts at 1750 rpm but the boat squats at the stern and we through a big wake. It just seems to me that there should be a design that keeps the nose down and doesn't waste all that energy plowing water. Can anyone explain this. Is it bad design or just the physics of a planning hull? Is the shat angle wrong?
The first picture is WOT at 13.3 Knots.
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Old 09-17-2015, 10:57 PM   #2
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Everything you have stated sound 100% to me and speeds are spot on for a semi displacement hull I doubt you own a planning hull
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Old 09-17-2015, 10:59 PM   #3
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Dave, it's a lengthy subject to explain what you're asking, but in a Reader's Digest Condensed version, here's what is going on. Here's a link to the topic of Hull Speed on Wikipedia. https://en.wikipedia.org/wiki/Hull_speed


In essence the low speed where the hull runs flat is at or near (or below) your boat's hull speed. At any speed above the hull speed you are outrunning your bow wave. It's the bow wave that supports the stern as the boat passes through the water. As you exceed that the bow wave moves from being under the stern to being behind the stern. As such, it can no longer support the stern so the stern squats in the water until you reach a speed where you're running on plane.


Now there are many on here who can give you a more scientific discussion of what I just 'splained in simple person talk, and probably some who will disagree with what I've said. To those guys--go for it.
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Old 09-17-2015, 11:56 PM   #4
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Sounds like normal semi-planing hull behavior.

I assume you have no trim tabs?
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Old 09-18-2015, 12:09 AM   #5
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I`m outside the class of adviser you want, but it looks like an SD/SP boat trying to plane, without success.
How about a hull width wedge extending forward 2-3 ft from the transom, tapering from a height of 1-2" to nothing. Would that keep the bow down by elevating the stern?
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Old 09-18-2015, 12:27 AM   #6
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Originally Posted by FlyWright View Post
Sounds like normal semi-planing hull behavior.

I assume you have no trim tabs?

Correct
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Old 09-18-2015, 12:29 AM   #7
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I`m outside the class of adviser you want, but it looks like an SD/SP boat trying to plane, without success.
How about a hull width wedge extending forward 2-3 ft from the transom, tapering from a height of 1-2" to nothing. Would that keep the bow down by elevating the stern?

Interesting, are you suggesting a transom at the waterline that would extend the boats water line by three feet?
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Old 09-18-2015, 12:34 AM   #8
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I know at about 10-12 knots, my ship rides with her anchor high in the air. If I deploy the trim tabs, I'll shoot up to 16-18 knots without touching the throttles. I've got a light planing hull, though, so it's apples to oranges, really. I don't know if tabs would help you or not. Fryedaze is probably too heavy for them to make much difference, otherwise she probably would have been launched with some.
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Old 09-18-2015, 02:44 AM   #9
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Interesting, are you suggesting a transom at the waterline that would extend the boats water line by three feet?
No, it`s hard to describe, try this. No extension of boat length involved.
Imagine a timber plate 1-2" thick, the full width of the transom extending fwd 2-3ft,and reducing in height over that distance to just a sharp edge, forming a wedge shape. The idea being: water flowing over the deepening wedge going aft pushes the stern up, and the bow down.
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Old 09-18-2015, 06:16 AM   #10
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To get the boat to plane all you need is probably 3-5 times the HP.
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Old 09-18-2015, 06:33 AM   #11
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To get the boat to plane all you need is probably 3-5 times the HP.



HP or torque ???? that leads us back to prop selection
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Old 09-18-2015, 06:49 AM   #12
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If you are truly curious about such matters, the naval architect Dave Gerr's "The Nature of Boats" is a good read for the interested layman. The Nordhavn-ized version of Beebe's "Voyaging Under Power" also touches on the subject of hull design fairly extensively, but doesn't discuss as many different design issues as Gerr's book.

I agree, trim tabs would be a great enhancement to the OP's boat, they're standard on virtually all planing boats.
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Old 09-18-2015, 09:33 AM   #13
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Your boat acts like our SD boat. We have a single Cummins 220. She quickly climbs to 8 knots around 1400-1600 RPM (depends on currents, so don't know exact number). From there, the bow starts to rise and additional RPMs don't add much speed, until we get to around 2200 RPM, at which point she attempts to plane. At WOT we can hit 15 knots lightly loaded, but we don't run at those speeds. In fact, the only time we get over 1800 is if we are in chop and want to level it out, or at the end of the day to blow out the engine. That or if some monster boat is about to try and cut in front of us throwing a monster wake just before we hit the no wake zone in our tight channel. That usually makes them slow down and get in line.
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Old 09-18-2015, 09:41 AM   #14
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normal behavior for a semi displacement boat. I suspect trim tabs would lift the stern and give you more speed but you don't have a lot of power to spare.
Bennett trim tabs are experts and will be happy to offer suggestions.


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Old 09-18-2015, 09:43 AM   #15
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the other question is are you reaching rated engine max RPMs? Look on the engine tag for specs.


Most travel lifts have gauges and can tell you the actual weight.


More that 30Kwould not surprise me.
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Old 09-18-2015, 09:52 AM   #16
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the other question is are you reaching rated engine max RPMs? Look on the engine tag for specs.


Most travel lifts have gauges and can tell you the actual weight.


More that 30Kwould not surprise me.
Rated WOT is 2650 RPM. We hit 2550. Someday I will get the real weight.
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Old 09-18-2015, 10:08 AM   #17
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Agree: investigate trim tabs.


You might or might not have enough horsepower/torque to make it a huge improvement... but OTOH, tabs might just lift you enough to get more reliably on a decent plane and stay there.


Probably save fuel, too, except for that whole "more RPMs than when puttering along" thing. Still, cruise at say 2200-2300 RPMs with tabs would likely be more economical than cruising at the same RPMs without tabs.


My best guesses...


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Old 09-18-2015, 11:10 AM   #18
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If the boat was light like Wayfarer's then trim tabs will have some possible benefits. But tabs on a heavy trawler w a hull more like a planing hull almost certianly will just create more drag. Fryedaze your boat looks like it's lighter than most trawlers so w more power you could plane. Since your Monk boat is lighter than most, fairly wide and flat over most of her bottom given enough power you could probably plane at 18 knots. I don't see her doing 25 knots gracefully and rightfully so as she is a SD hull .. but only slightly so.

Having said that what's wrong w the way she runs now? Can you see over the bow well enough? Only FD boats should run level .. and even they won't run level unless they are a knot or so under hull speed. The bow is for climbing up and over waves. That's why bows are higher than the rest of the boat. And surfing on following seas having the bow high (at least to some degree) is a good thing. When a boat runs above hull speed it runs most efficiently at a certian angle of attack. That angle varies depending on the aspect ratio (length to beam), dead rise, other hull shape differences like hard chine soft chine, bottom twist and where the weight is fore and aft and speed. To me your boat looks great at 13 knots. At 9 knots she's probably much more level and I doubt you run over 9 knots very often. Wake? Sure. Run a 15 ton boat at well over hull speed applying 420hp and there's bound to be a big wake. I don't see that you've got a problem at all but I'm just guessing re your pics and specs.
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Old 09-18-2015, 11:23 AM   #19
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Maybe Tab should chime in on this but In simple terms since water is incompressible boat speed is controlled by the physics that describe waves. From your top photo your bow wave has moved aft as has your quarter wave. The distance between the two crests is the square root of your speed divided by your boat's Froude coefficient( sqrt(13.3/F)). If you have enough horsepower you can put the bow wave on your quarter and your boat will level off and you can get on plane and break free of these constraints. There are some tweaks to hull design and props that can accentuate stern lift which may make this happen with less power but that's a different subject.


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Old 09-18-2015, 11:27 AM   #20
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IMO, you have a planing hull. But just like a GB, you need more HP to over come the drag of the keel, get it over the hump and truly on plane.

If you want to drop the now you need to add trim tabs.

Your boat is behaving normally for that style of boat with that engine HP.
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