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Location: Lakeville Ma high speed hull I'm in the planning stage of building my next boat.
Will you be building the boat or will you turn the job over to an exprerienced professional builder. Location: Lakeville Ma Flyer 15, a boat tailed steped hydro of the 40's, with a transom like the Stancraft Torpedo 29. Location: The Netherlands If you want to ride in rough weather, you might be better of not to use the stepped hull form, cause of the slamming that may occour.
Location: Lakeland Fl USA If you need to outrun redneck bass boats you are going to need something capable of well beyond 70 MPH. I suggest that you get Dave Gerrs Propellor Handbook ISBN 0-07-138176-7 before you select a boat design.
Location: Lakeville Ma I used the Castle Marine prop calculator and a fast runabout hull weighing in at around 800 lbs and spinning a 9x16 prop at 6000 rpm will get me (on paper that is) 84 mph. When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. I am told by the major international boat builder of this craft that their brand new hull designs require full negative trim on the twin sterndrive installation(-5') and most of the negative trim on the QL interceptors to achieve it's quoted top speed, even with very light load and fuel aboard.
My understanding is that these device adjustments should be for assistance in uneven or stern boat loadings, early achievement of planng speeds or to effect more comfortable ride in differing sea conditions. Location: Milwaukee, WI Trim tabs are often used to compensate for a bad design. All the hulls and decks are way over weight and internals made of ply and solid timber way oversize . So trim tabs are just another tool to add to the control of the boats performance along with the stern drive trim .
Some times we load our boats how they were never meant to be loaded , everyone loves to stand in the back and bee cool !!! All of my previous sportscruisers have always needed positive(leg out) sterndrive trim with no tab trim to attain top speed, and the original manufacturer instruction for this boat was only to use maximum negative sterndrive trim. Given the above information, do you still think it's an acceptable design status for your industry? Negative trim of a shaft drive is not the same as in a stern drive because of the difference in point of thrust application. My own boat i set 3000rpms and tweek the trim up and watch the spray out the side of the chine ,i can hear the sound of the motor change and sure enough 3150 rpms and thats it , hammer down and going for it .
Trim Tabs on a well designed hull allow the hull to run at its optimum running attitude over a broader range of speed, sea conditions and weight distribution.
For example if the boat can plane at lower speed and still keep its bow down cutting an oncoming chop it is essentially running on its lines and the good characteristics of its hull design are working. Calling Trim tabs a crutch for bad design (if it is a good design to begin with) is like faulting an aircraft designer for including flaps for low airspeed performance. A good designer likes to put all the heavy weights like engines, batteries and also possibly tanks close to the center of gravity, thus eliminating the effects of polar inertia, and to keep a good planing with no more than 6 degrees trim of the hull at full speed. It helps me to better understand what I've been seeing on vessels like the latest sport-fishers with enormous weight and power hanging off the transom. If the boat is running at a bow high attitude and you correct it using the power trim on the drive unit alone the propeller's thrust is now angled down, resulting in a lose of efficiency. On straight inboards keeping the propeller shafts closer to parallel to the water flow using Trim Tabs at lower speeds will also result in increased efficiency.

With bit of work, and the shipyard took out: the double berth, the cabins, the kitchen with microwave and electric oven, the big 120 gallons water tank, the big 80 gallons used water tank, the air conditioner, the genset, the big gas tank of 200 gallons -which was just at the stern-, the 250 pounds marine battery, the 2 engines plus a lot of useless things.
Morality 30 knots and a cost of exploitation divided by 3, with a boat as gentle as a purring happy cat. From the point of view of a NE these things are marvels of inefficiency with the diminutive propellers robbing power from each other. Youve done it all wrong, you decide what speed you want , then choose the prop to do it , then chose the engine to drive that prop.
Check out your local prop man he will no doubt tell you what you need and probably have a few for you to try.
Originally Posted by Frosty I would start with something in the ball park like a --9x9 for your 20HP. A 9x9 propeller would give a thrust about 600 N and require less than 6 HP at 6 kn speed and 1628 rpm shaft speed. With a 14x9 propeller the figures would be about 2000 N and 20 HP at 6 kn and 1150 N and 13 HP at 10 kn.
A 14x11 propeller would be 1900 N and 23 HP at 10 kn and 2800 N and 31 HP at 6 kn, thus close to at 10 kn but way too big for 6 kn. A 14x10 propeller would be 1500 N and 17 HP at 10 kn and 2400 N and 25 HP at 6 kn, thus a bit too small for 10 kn and a bit too big for 6 kn. I would not go to anything smaller than 14" in diameter, if you just have enough room (at least 5 cm from tip to hull). I think what we have here is an engineer who is accustomed to having an extra 25% runout of any adjustment beyond what should be needed. I'm not sure what 'philSweet' is getting at but I would state its obvious that you need no more trim than that required at WOT. Can you describe in more detail what happens if the drives are not fully tucked in and if the interceptors are not mostly down approaching WOT? Worth trying bigger, wider tabs as an experiment, doesn't necessarily create more drag either. I've uploaded several pics of a very successful one of the French Navy, a bit too big, but a good example of the results obtained when a boat is designed for efficiency and not fashion. The boat is very heavily built (Lloyds A100) as shows its displacement of 41 metric tons full loaded. Location: united states A boat that requires drives to be fully tucked in at WOT and interceptors down at WOT is unusual. Location: Australia Worth trying bigger, wider tabs as an experiment, doesn't necessarily create more drag either.
Originally Posted by Mr Efficiency Worth trying bigger, wider tabs as an experiment, doesn't necessarily create more drag either. Originally Posted by tunnels How can more surface area in the water not create more drag ?
Originally Posted by tunnels What is the better trim tab and wide one across the boat ?? If the trim is zero or very close to zero at this speed, there is only a small nuisance as full speed is not the longest situation. I note you all have crudentials, but with Ilan giving the designers the benefit of the doubt, with them maybe striving to achieve a cruising optimisation which prejudices the top speed trim ideal.
I never envisaged for a moment that it would also need near maximum negative trim tab with low loads!

It would follow that if it has been optimised for cruising speed, it doesn't work, and the situation is worse for top speed. Additionally since the propeller is now at an angle to the water flow its slip is increased, further reducing its effectiveness. It is quite easy to accomplish this by trimming the boat's attitude with the Tabs, and then using the power trim while observing the tachometer for best RMPs. LWL is about 14.6 - 15ft feet, draft about 2 ft, not sure of loaded displacement weight is but Im estimating this at 1000kg. Will a semi displacement boat (assuming it is) with this engine spec ever achieve above max hull speed of a displacement boat (5-6 knots) with same spec driven by a new 21Hp engine with this gear ratio ? What options might there be to get the boat above displacement speed by way of changing prop. At 10 kn it would have a thrust of about 300 N and require about 3 HP (much better efficiency).
At 6 kn it still would take more than 1500 rpm and have a thrust of more than 2000 N, thus it would not be worse than 14x9, if you just don't overload the motor. Good thing if you're designing a packaging machine, but hydro doesn't really degrade over time the way machines do (I haven't seen gravity break yet). But do you run your boat at maximum negative sterndrive and tab trim to optimise WOT and still not achieve a good pitch angle for maximum speed? Your comment about critising aircraft design is wrong as that's exactly what flaps are for, to improve low speed performance(like a boat) and your comment overlooks the fact that planes don't usually use flaps to correct wing profile at WOT maximum speed for a light load. My comment are about a boat designed very recently by a well known designer with an International boat builder. The question would be 1)has it got enough at WOT and 2) Is it achieved with a satisfactory balance of organic inbuilt hull trim as opposed to external devices. As you can see, there is nothing fancy but just optimization born from years of experience at sea. If you must direct drive the prop, then you will need to do some research into the shaft and prop system you can use to best advantage. There's no reason to provide more trim capability than that required at WOT with full tanks.
If so there's really no value in these trim devices, because if they don't achieve a relatively flat pitch at WOT with maximum negative trims then they'll be very little use to adjust for sea conditions. He consults me, desperate as he wanted to used it for chartering, needed to get back the investment and earn a living. If a good planing trim is achieved at cruising speed, typically trim is too low at top speed.
I have seen some sites which calculate these metrics but Im looking for an old sea dog with experience to comment on their opinion. If the specification is interceptor, then surely one that occupies a greater wide of the hull could be the ticket. Mostly trim tabs and low stern drive trim are needed while getting to plane or at (low) cruising speed.

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