# Jetboat hull reinforcement, HDPE or UHMW?



## squeekycurd (Jun 23, 2020)

Hi all, 
Long time lurker, new member. Ive spent the past few months swapping a Polaris SLTX 1050cc 2 stroke/jet pump into a 1975 Starcraft aluminum hull. I should mention, this is my first boat project. First boat ever actually, I am not familiar with how machines handle on water. Over the past week I've had the chance to run it on the water for engine tuning and found concern in the amount of flex in the hull under my feet. While the hull is cross braced in 3 locations and does not twist or distort in any way, I'm concerned about the thin aluminum sheeting when hitting waves. 

I've have found plastic is used to reinforce the hull from impact damage and wondered if adding an extra layer would benefit the structure of the aluminum. Good idea yea? 

If so, HDPE or UHMW. HDPE seems to be available from local building supply stores in 0.125 or 0.25 thickness and is relatively cost effective at $100 for 4x8 sheets. UHMW on the other hand is harder to get and holds a higher price tag. 

Any other thoughts? Am I overly concerned about the 'floor' of the boat wobbling around under my feet? would aluminum reinforcement be a better idea? 

Thanks all, looking forward to seeing opinions on the matter.

Tanner

(Trying to figure out this image thing)
https://imgur.com/8FThIpE
https://imgur.com/BN7NOz6


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## CedarRiverScooter (Jun 24, 2020)

That is a cool rig!

I recommend you just use foam board on the inside of the hull to increase rigidity. You may have to shape it some. You can get a hot knife from Harbor Freight for cheap.

I used locktite roof sealant caulk to glue it together.

Is the rear dam for sound abatement? I used to wear ear plugs when I had a 2 stroke Polaris, It put out 100 dB.


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## squeekycurd (Jun 24, 2020)

Thanks for the reply CedarRiverScooter! The rear dam/wing was installed for both sound control as well as water control. When I was testing the mechanical assembly without an engine cover, I let off the throttle quickly from a low speed which caused a good bit of water to enter over the transom. I figured when the engine cover was installed, that water would now be on my back. 

Ear muffs are still required, I will be testing sound deadening foams around the engine soon. 

This foam you speak of, is it Foamular or whatever brand of insulating foam used on homes? If so, its quite rigid, how would one go about shaping it to fit the curved profile of the hull? Do you happen to have any examples of this? 

Thanks!


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## CedarRiverScooter (Jun 25, 2020)

Yes Foamular. I used it in under the floor of my boar, as did many others. 

For your V bottom, I would get 1/2 inch thick & cut the profile for that height off bottom. You could 1st lay down strips lengthways to get it closer to level, leaving couple inch gaps for water to flow. Several more layers of 1/2 inch would be cut larger to fit the full inside profile. 

A hot knife would cut the foam quickly, but you could also use box knife or oscillating tool.

I would use urethane caulk to glue the layers together (doesn't take much) but not until you are sure they fit!

The floor will be flat. The foam board will need a rigid cover, such as plywood or aluminium. 

Make brackets to seat frames or ribs to hold the floor down, it is best if floor is removable for cleanout. 

Leave a 1/2 in.gap between the rigid floor & the hull so it doesn't stress crack. 

To make foam follow the compound curve of the hull would take a spray-on product, can't help there as that is above my pay grade.


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