Need to gain 2-3 knots

That's fine for now. The bow-up attitude is exactly what to expect with an overweight hull with a short length compared to its beam. It leads to a very high hump resistance, which you have been (unsuccessfully....) fighting with steep trim tab angles.

The critical characteristics determining the hump behaviour is the "slenderness ratio", ie the relation (waterline length/displacement^0,33). When this ratio goes below about 4,5 (in metric dimensions), there is a very sharp hump ridge, with a resistance that is higher than the full speed resistance.

This is why I suggest that you check every corner for unnecessary weight, and that you make the planing bottom longer (without increasing static buoyancy aft). If more lift is needed to pass the hump, it is better to go for interceptor tabs, since they have better lift-to-drag ratio than standard trim tabs (but that's a later story).

These are the primary steps, next you go for a better prop, but it's wise to work in this order, not the other way around.

 

Thanks. I'd never heard of interceptor tabs before, but looked them up. I a run to the boat last night to measure waterline beam, and it is about 86-88" at roughly midway from bow to stern, and about 77" at the stern. These measurements are a bit rough, +/- 2" or so. Okay, this is what I take away from all the above. Task #1: remove all unnecessary weight. Task #2: extend hull length with some type of cavitation plate (non-bouyant). Task #3: Try reworking existing prop or get prop with larger surface area (maybe a 4 blade with less pitch but cupped). Task #4: Move rudder aft into the cavitation plate in order to accommodate a longer shaft, and install new strut, shaft and larger diameter prop. Test performance after tasks 1, 2 and 3 to decide whether it is worth the expense of step 4. Did I get it generally right?

 

Your performance expectations may be exceeding the capabilities of the hull design.
The rounded shape of the bottom is not the best way to get a heavy narrow hull to plane on low power.
I would like to see more pictures, bow, stern, and side views to look at the deadrise and the waterline.
You might add to your task list to consider a higher gear ratio to spin the prop faster.
#1 and #3 are the starting points, #2 and #4 are a shot in the dark at this point.

 

The bottom is not really rounded at the stern, but has a bit more rounded section forward compared to later year hull shapes. Still, the original specs on these boats were most definitely planing hulls obtaining relatively high speeds. See attached. Even if these speed numbers by the manufacturer were overly optimistic, there is no question that the hull can plane and achieve plenty of speed with with the right setup/ weight/ etc. The biggest problem must start with the added weight of my boat, which may be 1,500 lbs or so more than production weight. Also, I'm guessing that the original drivetrain with gas engine probably had closer to a 1:1 gearing, spinning a 13 or 14" prop. With the diesel and current gearing in my boat, it is clear a larger diameter is needed. Also,my current prop has a very substandard tip clearance of only about 1/2" or less from the hull, and this may also be a big contributing factor to the poor performance of the current setup. I realize now that even the strut mounting plate itself blocks or disrupts some waterflow to the prop due to such substandard clearance, not to mention the disturbance caused by proximity of blade tip to hull. I suspect just increasing prop tip clearance alone would make some difference. If changing the gearing were possible with the existing transmission, I would consider that too, but to my knowledge it can't be done without swapping out the whole unit. Haven't checked on that yet to be sure. Gear is a Twindisk MG502-1.

 

There is another possibility not mentioned here yet. I don't know much about jets, so can't advise about them.
Might be expensive, don't know that either.
Anyway, if you were considering changing the red gear, the shaft, strut, propeller, and moving the rudder?
swapping for a jetdrive powered by your current engine might be economically feasible. maybe recuperate some of the cost selling the removed gear, shaft, prop, strut as a used package.

 

The boat probably started out with a gasoline engine that turned quite a lot more rpms that the diesel does. Prop diameter is not the problem, shaft speed is.
There are gobs of old 502's available, and a zf 220 will fit in as well.
All you need to do is get the correct trans ratio mated to the right prop.
Or rip it all out and bolt on a light, powerful outboard!

 

Or convert the diesel to a gen set and bolt on several electric outboards

 

Yes, that's about it, but skip the #3, it is a waste of time and resources. Since we know the performance of your propeller with "todays" boat, its performance with steps 1+2 will tell what comes next. Better to go directly 1+2+4 and then settle for the correct propeller diameter.

The 14x17 propeller is heavily cavitating due to overloading; it is working with very high angle of attack, which will trig suction side cavitation. Increasing the shaft speed and reducing the pitch (as has been proposed) can not compensate for the high loading, so changing the gearing to increase the shaft speed is no solution! The tip speed (and thus cavitation)will increase with rpm, still trying to keep thrust loading constant is impossible.

As to the suggestion on waterjet: WJ is, in fact an "underdiameter" propeller, with just the problems we see here. High loading at low speed is the real problem with WJ:s. There are cavitating WJ:s with problems coming over the hump everywhere, it's the most common complain there is with this propulsion type, so don't even think waterjet here!

Please let me do my favourite nagging stunt (don't take it personally TTT...):

How come that motorboat owners can invest seemingly unlimited amounts in engine power, in flashy electronics, in blablabla aso, but when it comes to the one single component, that converts all the power produced by zillions of precision paraphernalia in the engine, they are prepared to take whatever looks like a propeller, only it is cheap enough!!!???

The cost of a good propeller, suited for the operation at hand (and we can calculate propeller performance right close) will cost about one to two percent of the engine cost, or about the cost for one or two bunker fillings, or less than five promille of the boat. And a wrong propeller selection, as in this case, will completely ruin the boat performance; the loss in efficiency is consuming one tank out of two or maybe three.

I just can't understand it, this situation comes up over and over again in the posts on this forum, and everywhere someone is suggesting that "we'll get what there is to find on the scrap shelves that is cheap enough". This is really an issue for all of you hull designers out there; don't get lured, this is the case where size matters!!!