Boat Twisting?

[MADDOG355]

New member here looking for some help. My friend has a 18' Mantra (I think) SBC with berk pump, loader grate, rideplate, droop, and place with red insert. The boat has been great all season till our last trip it started twisting down on the driver side (looks like it is to flip over) any ideas? Nothing has changed as far as weight or setup so we are lost. We have noticed that it seems to be leaking a little water around some of the bowl bolts at speed but that is about it.

[Wicked Performance Boats]

explain better. BL

[MADDOG355]

At speed the boat looks like the driver weighs 2000 lbs (he might be 170 soaking wet). That is about the best I can describe it. When you nail the throttle at speed the boat feels like it is trying to push the bow down and twist the boat to the left (opposite of engine rotation) so it makes no sence to us.

[Boatcop]

At speed the boat looks like the driver weighs 2000 lbs (he might be 170 soaking wet). That is about the best I can describe it. When you nail the throttle at speed the boat feels like it is trying to push the bow down and twist the boat to the left (opposite of engine rotation) so it makes no sence to us.
If the engine power (torque) isn't being fully transferred to the drive (pump in this case), it will transfer to the hull, causing the "twist" you describe in the opposite direction of engine rotation.
It usually happens when a drive shaft breaks, shaft slips out of the coupling, or a prop fails of falls off.
No idea what would cause that on a jet.

[Moneypitt]

Look under the carpet and see where the hull is flexing, and DO NOT "TEST" IT AGAIN..........until it is totally inspected. Sounds like the jet is trying to carry the nose, but the hull won't hold the nose up......Scary at best....MP

[BLEWBAYOU1]

Check things like rotten stringers, Loose motor mount bolts or even cracked hull. Also check that the bulk heads have not broke loose of the hull on the inside under dash.
Brian

[MADDOG355]

The boat will do OK if the diverter is all the way up but that is the only way it will act right. (this all started after getting cought in a storm and he might have hit a log but he was not shure) we checked the bottom and didn't see any damage. The stringers are bare (painted black before he got it) we have not noticed any cracks but will do a good inspection in the future. (the season is over here so we have plenty of time) If the stringers are bad how much trouble are they to replace?
and thanks for the help!!!!!

[Duane HTP]

When you say it is leaking around the bowl bolts, does it have a blown bowl gasket? Is the bowl to suction outside of the boat, (set back pump)? I've seen a bowl gasket blown on the lower side of the pump do just what you are talking about. The force of the water blowing out was pushing the boat sideways.

[ONAROLL]

Check things like rotten stringers, Loose motor mount bolts or even cracked hull. Also check that the bulk heads have not broke loose of the hull on the inside under dash.
Brian
If this is one of the 300# Mantra tunnel hulls, gunnel supports placed midships will help stiffen it up, look down the sides on top of the hull cap and look to see if the gunnels sag inward, also as Brian said check the bulkhead, these boats are really light, I went through this and I can put you in touch with the guy that convinced me........Vic

[Dan Lorenze]

I had a boat that twisted real bad whenever I hit a small wake, it turned out to be a rotten floor all the way through the boat. I replaced the floor and bulkhead and rode very solid. Like others have said, maybe stringers or motor mounts..

[SmokinLowriderSS]

Yes, check the structural reinforcements, ALL of them, the floor, the stringers, and the cross-bulkheads. Without these being tightly glassed to the hull (and being structurally solid), all you have ia a large bowl of plastic which can twist in all sorts of ways. Replacing them is not so bad, is messy, but can be done with reasonable ease.
Best of luck to you.

[Moneypitt]

MD355, be sure to come back and let us all know what you find. Issues like this can be very educational to all, and just might enlighten someone else prior to the "unattended" result. These boat are alot of fun and sometimes the danger aspect is overlooked...............MP

[hallettman]

maybe the foam floatation under the floor is retaining water. i say this cause that was my problem last year.

[MADDOG355]

The bottom of the boat if bare (no floor or foam) just stringers but there is no reinforcement between the stringers from the motor mounts till the bulkhead under the dash. I am thinking it should have one infront of the motor (where the back seat would be). It has had bulkheads replaced infront of the saddle tanks (becouse it was folding in there). so I am guessing the stringers will be the problem. I will try to check some of this this weekend.
Agian thank you all for the help I will keep you posted on what I find.