Mustang V1700 boat, with a 1981 Johnson 90hp, & a 9.9hp Suzuki on it (new to owner).
1 - Took it out today, it sat low at the transom with two people forward & one person at the stern.
2 - Even with just one person, viewing the rig from land or elsewhere, it was sitting low in the stern,
3 - The water was covering the exhaust relief holes even if I was moving.
4 - A deep V Hull. No knowledge of water in the sub floor, 3 bung holes in the back, 1 for the top floor and 2 for the sub floor,
5 - The motor would not hit full throttle. It would go very slow but move without issue but would go over a certain speed don’t know the RPM as it’s not working on this boat.
6 - Not sure if I should raise the motor and just use the existing holes just move it higher or if there’s something else I could do, I have a 60l steel tank under the floor but it’s right in the middle of the boat.
*I edited the authors quote for clarity, above.
Out of gear, at an idle, and at various rpm not to exceed 1800rpm, do a drop test... pull the spark plug leads off the spark plugs one at a time to check the response... the drop in power should be even on all cylinders... is it? Making a note, keeping track of what spark plug came from what cylinder... do all spark plug electrode areas look identical... what do they look like (Pictures?). Spark plug Make (Champion/NKG/what, Designation (UL77V/L77JC4/QL77JC4/what?) Type (surface gap/electrode gap?),
#1, #2 & #6 - I would assume that the engine did not sit low in years past, and that the engine is set at the proper height. I strongly recommend that the engine be left in its present attachment holes for the time being at least as, to me, the bilge of the hull would be more subject to this low setting.
The two holes mentioned are exhaust relief holes. Inside of that long housing between the powerhead and the lower unit is another like housing. The exhaust passes thru the inner housing. The area between the inner and outer housing is filled with water to cool the outer housing otherwise the paint would burn off... Those two holes allow the excess water to escape otherwise water would enter the lower cylinders via the exhaust ports.
#3 - Water covering those two holes creates a back pressure to the cooling water, possibly allowing water to access the bottom cylinders, perhaps in very small amounts but still... those holes should remain clear.
#4 - I have no idea what a sub floor or a top floor ID would pertain to. The area that one would be walking upon (sub floor?) would be the "Deck"... the (top floor) could be a fishing look-out "Tower?". The model number of the boat (1700) registers in my mind as being 17' long, a runabout.
#4 - The ID title you have assigned the holes at various places of the craft... I am well aware of the description that would relate to such a title (AH) and have observed a few boat operators during my years in the marine trade that would indeed fall into that category... However that ID, title, description, whatever has no place within a boating crafts manual. If what you're speaking of is what appears to be a brass tube thru the transom, a bulkhead, whatever, that would simply be a "Drain Plug Hole".
#5 - With what you have described... sitting that low at the stern, it would be impossible for the engine to develop full power even if the engine was in exceptional good condition and supplying full power as when throttle is applied, it is natural for the stern to drop first, than as forward thrust is applied and speed gained, the stern would rise with the boat planing. Your explanation of the boat leads me to believe that the bilge has much water in it and as throttle is applied, the stern naturally drops, but in doing so, the water (all of it) rushes to the stern adding to the weight.... In essence, and reality, the engine is trying to push the stern of the craft "up" instead of forward and cannot escape that ordeal.
#6 - As already entered above, I'd let the engine mounting alone as it's possible that previously the rig behaved properly, and now is not due to a hull condition. Personally I have never seen a boat during my years in the trade that did not have a way to drain the bilge, either via a drain plug at the outside of the transom at the bottom of the keel, in your case the "V" bottom of the transom. or being equipped with a bIlge Pump" located strategically to be most efficient. That craft being a 17' runabout, I would think a drain hole/plug at the center botom portion of the transom to drain the bilge would exist.
The times I've come across the problem you've explained were due to either... trying to plane a 21' craft or larger with a 35hp, a flawed high hp engine, a hull coated with marine life, or a water filled bilge. Personally I suspect the bilge as no boat I've ever come across was perfectly sealed, water always had access to the bilge... somewhere!
