Cat Sponson Modification idea to improve fuel economy

I own a 22 Seacat Catamaran and this is a pic of her rear sponson.
The hull is getting about 2 mpg at a 25 mph cruise, and light boat tests claim she should get 2.8 mpg with the 2002 Suzuki df115 hp 4 strokes.

I feel she needs more planing area to improve her fuel milage and I would like to glue and screw 4 pieces of SS316L 90 angle to the upper chines shown in this photo ib the black bottom paint area on the inner and outer chines of each sponson to increase their width and effective planning area.

I am contemplating using SS316L 3" x 1/4" angles glued to the sides of the sanded hull with 3m 4200 using #8 x 1/2" long screws on 6" centers.

My hull weighs about 5500 lbs drafts around 12" at rest, so I am hoping that these 4 angles with a combined 13.3 feet squared of planning area moving below the water surface at an attach angle of 3 degrees will give me about 2000 lbs of upward lift at a 25 mph cruise and hopefully net me a 20% improvement in fuel economy.

Can anyone help me with this calculation to see if I am close to the one ton in lift and perhaps also comment on the feasibility of this idea?

Thank you

 

She's a heavy tub if 5500 lbs. Those engines would be marginal power, probably. Looking at another picture of these boats, the hulls look quite "fat", leaving a quite narrow tunnel. I would be loath to drill any holes into and through the boat hull, below the waterline. And if your angled metal doesn't work, you have a collander. I am concentrating on the design of the hulls below the chine level, they have quite insignificant chine flats, which is not ideal, imo. If someone put a gun to my head and told me to "fix" it, I'd be more inclined to fill in below the chine to give nice wide flats, inside and outside of both demi-hulls. You could test it with temporary inserts before committing any further to a permanent arrangement. You could even go the whole hog and look at planked bottoms to the demi-hulls. But it is all a lot of work, and you will run the risk of no, or not enough, improvement, and a boat that buyers might steer away from because it has been fooled around with.

 

Stern view of the hull :

 

Illustrating (shakily with paint program) what I was talking about............

 

Yes she is over built and short in length losing needed displacement. The dry hull alone is around 3200 lbs which looks to be about 1000 lbs over what the best builders have reduced their hull weights too 20 years later. But she will still do around 39 mph on the 115's, and lots of owners pair 140-150 hp motors on them. The other problem is that only the rear 1/2 of her length is in water at cruise reducing planning area and her ability to track straight, but also making for a very dry hull.

Your thinking matches my original thoughts of adding tapering flat pads to the bottom but as you say this is a lot of work and anything done to her bottom will effect resale. I might add that there are 2 other chines on the hull that you see due to their small size. The additional problem with the pads is that they are in contact with the trailer and would need to be stronger than the angles.

I had also thought about using foam to fill the chines to a lower wider demension, but this only makes them slightly wider and the chines extend to the bow so again a lot of work for a small benefit.

Hopefully, someone can do the force calculation requested to see if additional outer chines are something that adds significant lift, starting with the example given for 3" x 10' x 4 pieces at 3 degrees x 3 x 12 mt per second. I need some numbers to quantify cost benefit.

Thanks

 

Bullshipper, in order to increase fuel economy, you need to decrease drag. Lift may or may not have anything to do with that. Lift only helps if the lifting surface reduces drag elsewhere by more than it creates. So asking how much lift the angles may produce is getting the cart before the horse.

Do you have a good table of fuel rates, rpm, and speed?

Are the props counter-rotating?

What is the prop specification and have you tried any different ones?

Are you willing to give up some top end to get a better economy at 25 MPH?

 

Good points Phil thank you.
Here is a link to the boat test done on my actual hull.
http://www.suzukimarine.com/Product...oats Inc/Sea Cat 22 Center Console Twin DF115

With my now older motors and a ttop my speeds are 10% less thru the test light boat rpm ranges, ,but my fuel economy is about 26% worse. This leads me to believe that the slip on the prop has suffered a 10% loss but that the motor is using 26% more fuel to get to the rpm level. The props will reach 5900 rpms at wot where in the test 6000 was eachieved, with less fuel as the motor limiters kicked in, not the end of the throttle cable.

I believe the attack angle of the top may be a little to inclined, and catching air when the bow rises on plane, increasing drag but, the hull also seems to wallow in the trough at less than 30 mph, especially with a full fishing load.

So I thought that my idea of adding a poor mans foil might help when a lot of load is applied to get more of the sponson and its wetted surface out of the water.

I do not want to put a foil inside the narrow tunnel as I believe it will make the hull less stable lifting in the center as opposed to its outer chines.
Here is another pic of the hull with its ttop shown this time.

 

If 5500 lbs is correct, the difference is right there, between your boat and the test boat. You can't expect to have another 1200 lbs in it and no difference in fuel use. I note the boat has lift devices fitted to the outboards, and all the pics of these boats show a noticeable stern-down attitude underway. If your boat is over-built, and the extra weight is biased toward the aft part, naturally there is a penalty to be expected. I would be disinclined to modify the hull in a way that cannot easily be reversed. Your original idea is worth a trial, I think, just with glue, no drilling. I take it you mean like this:

 

You could try just putting a pair along the inside, and test that to see if it makes any difference. What it should do is dampen any porpoising tendencies, and lift the stern. If you notice an improvement then do the outside ones, that way you will have less to clean up, and in a less conspicuous area, if the inside ones make no discernible difference !

 

I have seen a few different planing power cats with rounded demi-hulls, with little or no chine flats, and they have not generally been well regarded. The tendency is to trim nose-high underway and bounce and porpoise, and be power-hungry. In other words, inefficient. The successful boats have hard chine, straight sections, wide flats, and usually plank bottom demihulls. Reason for that is the effective beam and bottom surface area is not great, and going to rounded sections just increases the ill-effects of that. Pack in a lot of weight, and it gets worse again.

 

Thanks for the data, it's a rare and precious commodity these days.

So I took a very quick look around. The reported figures for the test boat show a slip of 20 ft/sec at 4000 cruise. This is already a bit high for economy cruise, but not out of line. However, if your speed is down 10%, then the slip jumps by about 20% and this is a number I think is worth trying to improve. It looks like you can hang 14.25 X 20 SS props on that motor, and I would give them a shot. And I would run a test w/o the top to see if that is worth fiddling with. It could be pitching your nose up. If you are already trimming nose down, and now have to add even more nose down trim, that could show up in economy.

But in the end, the weight increase is a problem without an easy solution, and you oughtn't expect to match the fuel rate when you are 30% heavier than the tested boat. Good luck with it.

 

The avoirdupois situation makes any tinkering unlikely to produce improvements other than small ones. Maybe there is weight in there that can be shed, could it be there is water-soaked foam hiding in there, it is a fairly old boat. There should be inspection ports to give access to both sides, to establish the situation. And if there is no foam in there, that is another problem. In Australia many boats were built with no flotation foam, I suspect in the US the foam will be there, if so I would be checking for moisture.

 

Yes thats it exactly. Just have to find a way to clamp the angle iron to the hull while it dries instead of using short screws.

But I do need some promising numbers before proceeding. Will the pads add at least 1500 lbs of lift to decrease the wetted area and its drag.

If anyone know's of a strong adhesive suitable for under water use that will dissolve with a low base solvent. 4200 and 5200 are very permanent and will remove gel coat if pried off in case this doesn't work.

 

I agree especially on a longer hull. The smaller ones like mine do have a softer ride on rounded bottoms and also grip less on turns. I would prefer a thinner sponson with a 2-12" wide delta pad or even a V hull bottom on the sponsons with 13 degrees of deadrise.

These Seacats have a lot of cross bracing in the sponsons and over the tunnel that adds to their overall height. Add that to thick resin rich coring and you have a hull the runs and lasts like a brick crapper.

 

No foam or wood in hull, except small amount of foam around the fuel tank to further secure it under decks. I won't buy a hull with foam under decks for the reasons you mention. Her 5500 lb weight checks out on road scale (adding another 800 lbs for the trailer) to original specs after adding to dry hull weight +motors + fuel and ttop weights to get total. So hull water is not-foam is not the problem.

I could remove the material on the shade, but its hotter than blazes here so making the shade larger not smaller, is desirable. Its angle could be changed to catch less air, but again my gut tells me to add angle foils using water not air. Poor man foils with 4x the foil's area.

Lifting skegs could also be used but I will probably be laughed off the block with just angles.