Porpoising Ski Supreme

[SABoater]

I have purchased a 1980? Ski Supreme that runs really well, however at about 58 km/hr it starts to porpoise. Having had and driven many inboards in the past, I can not understand what is causing this. There is no water in the boat, 3 seated adults in the driver and observer seats with a full tank of fuel. I believe that a trim plate on the transom will help (as well as reduce the wake for slalom) but these boats aren't supposed to Porpoise!! I would like any suggestions and or help - maybe there is something I am missing.

[marshmat]

Is it propped correctly? Check with the builder to see what prop it shipped with. Also check if the prop is damaged.
Simply because there's no water in the bilge, doesn't mean there's no water in the boat. Some older flotation foams can deteriorate over time and absorb water. If this has happened, the boat could be really off balance but still look dry. If the boat has a lot of weight in wet foam, this could be part of the problem.

[messabout]

Older boat may have some bottom features that cause the porpoising problem. This kind of problem is not uncommon. Age, use, and particularly the way it may have been stored influences the shape of the bottom.

Check the afterplane for straightness. Look at the aft six or eight feet of the bottom. If the bottom or some part of it turn upward you will have the problem you describe. Boats that are stored poorly often get a condition that goes the opposite direction. A condition commonly called "having a hook" . In that case the boat will tend to assume a nose down attitude. Some boats are deliberately built this way and the downward slope of the afterplane is sometimes called "kick down". That design feature helps to promote planing at low power. In your case, look for any kind of rocker or belly in the bottom, possibly near the location of the engine. If you find such a condition, you may be obliged to use trim plates on the transom or resort to some extensive repair.

[SABoater]

Marshmat. Thanks for the advice. It is running a 13x13 stainless prop that seems on the face of it to be OK with no vibration. The floatation material is under the sealed in floorboards and difficult to access but I will cut out some near the transom and check for water content.
Messabout. The bottom looks fine without any obvious belly or rocker.

Thanks for the advice so far. The other option is adding weight in the bow but that will also increase the wake - not something the skiers will be happy about but worth a try. If weight in the bow works - would this translate to adding a trim plate on the transom as a permanent fix?
John

[marshmat]

Originally Posted by SABoater

Marshmat. Thanks for the advice. It is running a 13x13 stainless prop that seems on the face of it to be OK with no vibration. The floatation material is under the sealed in floorboards and difficult to access but I will cut out some near the transom and check for water content.

Easier to just drive across the weigh scales at your city dump, with the boat on its trailer. Compare against the design weight (don't forget the weight of the trailer). If it's a lot heavier it's probably absorbed water; if it's pretty close, then your problem lies elsewhere.

[SABoater]

Does anybody have an idea of what prop these boats came out with. I believe 12x13 but would a 13x13 make a huge amount of difference?

[gonzo]

No, it shouldn't. However, these boats were not designed for high speeds. 58KPH is at the upper limit.

[glassr]

SA Boater ,your ski supreme is primarily a competition ski boat . What I mean by this is that it was made to run a set speed with a nicely rounded wake for optimal skier towing. You would rather cross a wake that is rounded rather than a wake that is peaked while running a slalom couse.
This can be done by cupping the prop,and is probably the reason that you are having the porpoising effect as you approach the low 40mph area.Your hull shape chine angles and your transom shape are all factors also. I would take a look at the prop that is recommended for the hull, if your not too concerned about the wake quality try a prop with no cupping and a little less pitch

[Frosty]

I cant Immagine one of these boats porpoising, however a sand bag or 2 on the bow for an experiment would see if it was severe or not. The pich Dia and cupping are important as Is rake. A fixed shaft totally non adjustable propulsion system. I seem to remember them being equipoise cupped blades.

[Willallison]

I fail to see what your prop would have to do with porpoising.
Messabout and Jack are on the right track - the main reasons why a boat propoises are because 1. it has rocker in the bottom and 2. because the centre of gravity is too far aft. The real solution to the first is to buy a different boat. The second is (usually) a less drastic fix.
As Jack suggests, start by sticking some weight up the bow. If that fixes it, then start looking around for all the stuff that you can move fwd. The battery, ski's, fuel tank, etc etc. When this happens, are there people sitting down the back? Move 'em fwd.
The only other circumstance in which I've experienced porpoising (of a sort) is in an inflatable dinghy. At speed, the 'hull' used to develop a sort of corkscrew action - it would suck itself down onto the surface and develop a hook in the bottom, this would move aft until it would suddenly break free, before starting again. Most odd - fortunately not dangerous, but rather wet!
Sorry, I digress..
As has already been stated 58 km/h is as fast as you should need to go in a competition ski boat. I once drove a Mastercraft which suffered from the same problem - but not until it was flat out at a bit over 60 km/h.
This is a straight inboard isn't it?
And you haven't got a bladder or some other wakeboarding device fitted?

[Frosty]

Willi In the surface prop world a adjustment of the 'rake' of the blade will have considerable effect of bring up the bow on a fixed drive installation.
I think that this boat will have a slight power tunnel, or a full one i am not sure. It would seem likely that some one has reproped this boat for speed rather than for pulling. I think a standard prop would be a good place to start--along with a sand bag or two!!

[Willallison]

I'm not familiar with the ski supreme, but certainly none of the inboard ski boats that I've had anything to do with have incorporated a prop tunnel.
And am misunderstanding you? When you talk of surface drive, do you mean surface-piercing drive? For it certainly isn't one of those!
Perhaps SABoater could tell us - how fast does the boat go flat out?
As I said 58 km/h (or 36 mph) is the maximum speed a slalom skier ever runs at, so most ski boats won't go much faster than that

[Frosty]

No I know its not surface drive, 36mph is flat out for compettion but you will need extra gas to keep the speed accurate for comp work or the skiier will be disqualified. Plus a bare footer needs 40 plus --more like 42 and you dont really want to be blasting around flat out so yeah they should do 45mph say!.

[Willallison]

yes, of course - though I usually foot at 38 mph - at 42 the water hurts too much!

[Frosty]

Yes I know what you mean, but when you get to be a big fat bast*** like me you will need a good strong 42. There has been occasions when for some reason the driver took it up too much, boy it gets a bit hot down there.

I read once that these speed record bare footers inject thier feet with morphine before making the record attempt. Its difficuly to imagine bieng seriously burned by water. Any way these guys would readily accept the fact that they would not be able to walk for a month or more after.