dinghy flex

[Jack Long]

Okay, so after all my ridiculous hi jinx the dinghy/motor combo I am keeping is a 96ish Zodiac 310 Zoom and a 96 Mercury 15hp. The Zodiac is rated only for a 10HP but I feel like this will be okay. My harbormaster had suggested that 5hp over wouldn't be a problem and then, as a total coincidence, a guy a couple slips down got almost the same motor for his...

Anyway, sometimes when cruising along I can feel a lot of flex in the floor boards. Its like the whole boat is flexing with waves. The amount of movement between the rearmost floorboard and the one in front of it is pretty extreme.

Is this normal? Dangerous? A byproduct of too much speed/weight? The water is pretty flat when I am doing this and I am on plane.

Thanks,
J

[Minggat]

Is your dink pumped up "basketball hard"? I'd check with a local inflatable repair shop for a gauge and pressure spec. It might be all three, (low pressure, weight and hp). remove one and should make a big diff. But the only one that is adjustable is the pressure. I don't know about your Merc. But my 8 hp Evinrude is the same block as the 10 and 15 hp engines, so I am told. Which means weight there would be the same. That would narrow it down to 2 possible culprits. Checking/fixing the pressure would be the easiest/fastest/cheapest.

[Jack Long]

I am not at all satisfied that it is at an optimal pressure. I will see what I can do about that.

I wish the pump screwed into the valve or something. Trying to hold it in while using the foot pump is insanely awkward!

This motor seems to weigh the same as the 8hp I used before. So, your instincts on this are dead on!

[Minggat]

I'd also check with the repair shop to see if you have the proper connection. If memory serves me, I think they all just slip fit in, and should easily stay there.

[GordMay]

A “moderate” amount of flexure is probably normal.
Most Zodiac inflatables are rated to a maximum pressure of 3.4 psi (240 mb).
I believe that a 20% air pressure loss (to 2.72 psi), in a 24 hours period, is considered normal. If you have a problem with an air leakage, check all valves are intact and in closed position (nothing clogging the valve).

[ssullivan]

Floor board flexing is perfectly normal in a non RIB dinghy.

We had an aluminum floor in our old Achilles and it flexed with a 5hp engine on (and was rated for 10, I think).

Is it mostly the action of the floor flexing over the waves as you plane?

That's normal - especially if you have a folding floor.

Originally Posted by Jack Long

Okay, so after all my ridiculous hi jinx the dinghy/motor combo I am keeping is a 96ish Zodiac 310 Zoom and a 96 Mercury 15hp. The Zodiac is rated only for a 10HP but I feel like this will be okay. My harbormaster had suggested that 5hp over wouldn't be a problem and then, as a total coincidence, a guy a couple slips down got almost the same motor for his...

Anyway, sometimes when cruising along I can feel a lot of flex in the floor boards. Its like the whole boat is flexing with waves. The amount of movement between the rearmost floorboard and the one in front of it is pretty extreme.

Is this normal? Dangerous? A byproduct of too much speed/weight? The water is pretty flat when I am doing this and I am on plane.

Thanks,
J

[Charlie]

Have you thought about reinforcing the transom prior to having aproblem to account for the extra HP?

[Jack Long]

I have thought about reinforcing it yes. I am not really sure how to do something that would help though. I am not worried about the strength of the transom itself- but maybe how it attaches to the tubes. The other dinghy had triangular pieces of hyphalon in sort of a "brace" position but I can't imagine it actually doing much...

Maybe I need to find another one from around the marina to try. The trouble is everyone else seems to have an RIB. I was also thinking about adding an additional floor brace.

[Charlie]

I have a 310 as well. I would think that you could make the transom connection more solid by bolthing a brace from the floorboard to the transom. I know this would add another step to setting up the boat but with the extra HP it may be worth it. Please have no experience in boat design or engineering so I don't know if this may cause problems elsewhere.

[Cheechako]

All i've had seem to flex badly at times. Most 15hp are same weight as the 10hp so I dont think that is an issue. RIB is the way to go.