Modify the old hull?

[JetSled]

I have an opportunity to repair my boat in an aluminum welding shop to repair the hooking and dents on the bottom of the hull.

It has seen 20 years of use and has always had issues of gulping air in choppy water... leading to cavitation. By use, I mean jetting rivers... usually moving into shallow water on every trip.

I would like to get recommendations from those of you with experience.. hoping you can provide suggestions on what would be possible, feasible, desirable on this hull.

The shop where work will be performed is experienced in aluminum welding, not boat hulls. They have the equipment necessary to rotate my boat upside down and perform all welding necessary. It will only cost me materials and a fishing trip!

This was a custom made 1980's flat bottom, on which the original builder added a hollow center section, presumably to soften the ride or absorb the dents..? May have been added to divert air away from jet intake?

The softball sized dents may be repaired.. but I primarily want to focus on getting the centerline/bottom straight.. and improve the flow of water into the intake.

Let me know if you need more info or pics...
Here are some pictures:

The intake grate has been removed... I'll find a picture the grate installed.

 

[JetSled]

Re: Modify the old hull?

Here is the intake grate from about 5 years ago... I have since added some length to the forward forks, and welded in 4 more bars to prevent large rocks from being ingested.

 

[JetSled]

Re: Modify the old hull?

Here is a sideview of the hull

Here is the transom

I have fixed all the problems I can, but the inherent 'bad design' of the hull is going to be tough to overcome. I use the boat 95% for fishing the Fraser River... so it doesn't need to be a performance gem. It is what it is, I understand this. I am exploring whether there is any way to pursue improvement of the hull to rid the boat of cavitation in choppy water...without dropping the jet and engine down 3 inches... or completely replacing bottom.

I envision cutting out the middle section which was added... and running a flat section directly into the intake. This will give me a 3-4 foot tunnel.

Any advise is kindly appreciated .

 

[JetSled]

Re: Modify the old hull?

The addition to the bottom completely fills with water while boating.. it drains out fine while trailering the boat home.

 

[ondarvr]

Re: Modify the old hull?

The hull bottom looks normal (beat up) for a jet boat and I don't think the sucking air problem is due to the dents, its more likely just because of the flat bottom with a little help from the recessed intake design. A slight V will move the air away from the intake even in a light chop, but yours is almost flat. I wouldn't expect a big (if any) improvement with a new bottom.

 

[JetSled]

Re: Modify the old hull?

Is there any value keeping the section which was added? Why would it be there? I could remove it and add a few strakes to keep the boat straight...

The bottom is almost flat.. here is an older shot of the transom.
Note: the transom plate was removed, as it added lift to the stern which worsened cavitation in chop.

If improvement is out of the question... is any advise at all?

 

[drewmitch44]

Re: Modify the old hull?

What transom plate was removed? In your last post you said the transom plate worsened the condition. Which are you talking about? Just curious. Thanks.

 

[oops!]

Re: Modify the old hull?

ahhhh yes......another west coast whacko...!!!!.....

375 hp running the river.....lol

welcome to i boats.....we have quite a few guys in your area.

ondarvr is another jet boat guy ....with huge hull design knowlage....reade that one twice.

cheers ....and again ....welcome
oops

p.s.......sorry bout the vancouver giants ..........lol

 

[JetSled]

Re: Modify the old hull?

Here is a picture of the trim or transom plate which was removed... It mounted between the aft bars protecting the jet bowl.

 

[JetSled]

Re: Modify the old hull?

Oops... were hoping good things for the Canuckle heads this year!

I've been boating all my life, started with a Gary 14' sailboat at 10 years old. My dad moved to an 18' Thunderbolt (single screw ski boat), then a 30' twin screw Chriscraft, and last a 36' Monk with a 555 Cummins. I had a 22' Sea Ray then downsized to my current jetboat in 2002. I ran a 14' seadoo jet for a few years with a 175 merc sport jet.. that was a boat which was great on fuel!

This boat was purchased prior to me studying the fine art of jetboating. If I knew then, what I know now, I would have purchased a radius hull which provides better turning and no caviation in the chop...

I have run this hull without regrets... but it sucks when the wind kicks up and I can run her on plane without gulping air... a major drawback! I have the time and means to improve the hull, so why not? I would like to at least add a 2 foot tunnel to improve laminar flow to the intake.

I do like the hulls ability to navigate in skinny water where more boaters (fishermen) fear to go

Cheers to boatin!

 

[dreamchaser]

Re: Modify the old hull?

hey jetsleder, nice looking machine , think i seen it on the fraser screaming by! hope someone will pipe up and give u some sage adivice

 

[ondarvr]

Re: Modify the old hull?

Extending the tunnel may help, but if air is reaching it now, extending it further foward will just mean it will be easier for air to reach it.

As you found out, flat bottom jet boats suck....air, not a lot you can do about it. About the only thing that will help is changing it to a 6 or more degree V.

Adding a couple of strakes will help tracking and turning, mine has two and I may add two more.

 

[JetSled]

Re: Modify the old hull?

I appreciate your reply...! Flat bottoms are awesome in flat water... and totally suck 'air' in the chop!

 

[JetSled]

Re: Modify the old hull?

After some thought my plan is to extrude a tunnel 35 inches forward of the intake... and pop it open to 11.5 inches in width... I got the formula from an article here:
http://www.outboardjets.com/boatSelection.php

The article covers outboards... but loading an intake should be the same for inboard jets... no?

The magic words I heard were:
"The top of the tunnel width should be about 1 ? times the width of the jet drive water intake. The tunnel length doesn't have to be longer than about 2 ? times the water intake width."

Since my aforementioned sub-bottom fills with water currently.. I won't loose any buoyancy... and the modification should load the jet with better laminar flow water/bubbles/sand or whatever is running under the hull. The current transition is sharp and DOES NOT load the intake properly.

Strakes will be added... any words of advise or reference to other posts are welcomed. How long, where is best placement, shape?

I appreciate the attention. Captured wisdom will be put to use

 

[ondarvr]

Re: Modify the old hull?

This is the paragraph right before it lists the dimensions for the tunnel.

A properly designed tunnel, combined with a slight V bottom hull can greatly enhance jet boat performance. It should raise the motor 2-3 inches and place the heel of the jet intake flush or slightly above the bottom of the boat. A jet tunnel doesn?t work well with a flat bottom boat due to air ingestion. It?s imperative that the hull is designed correctly with the tunnel for the boat to operate properly.


Like I said, your hull isn't a good candidate for a tunnel in the first place and making it longer (larger) may just create more issues.

Not that many builders use tunnels because they're difficult to get right and the whole hull needs to be designed for a tunnel.

 

[JetSled]

Re: Modify the old hull?

Hydrodynamics is not a strength of mine, but I did study it in college.

I will let you know what I change, and advise how it works out... I do appreciate (and understand) your words of warning.

Any work performed can easily be undone if I get negative performance/results.

I believe a few minor changes will improve impeller loading, but won't resolve performance in choppy water. Vacuuming deeper water is always better for a jet in the chop. Noted with thanks.

I have a big spoon on the bottom of my boat right now.... this causes some diversion of air from its torpedo section. Air bubbles going down the center, will continue down the center if I install a 2 foot tunnel. This tunnel will also likely pull surrounding flow into its stream due to its negative pressure.

The work may harm more than heal... I wish I was good at modeling so I was able to perform small-scale tests.

 

[Bondo]

Re: Modify the old hull?

with huge hull design knowlage....reade that one twice.

Ayuh,.... I Did,... It was Mis-spelt both times.......

I'll preface by saying,... I know Only enough about Jetdrives to be Dangerous......

Wouldn't redesigning your finger grate, into more of a loader grate, do what you're hoping to do by changing the hull bottom,..??

I know you play in Skinny water,+ it's a Fine line between water,+ Rocks,...
Just seems like you outa be able to scoop more with the grate...


Btw,... Welcome Aboard,....
And,...
If you resize your pictures to 480x640,....
We wouldn't have to be sidescrolling to read,+ see your thread....
You'll get much more traffic,+ replies if folks like Me don't have to hunt for the buttons.....

 

[JetSled]

Re: Modify the old hull?

The grate forks were modified about 4 years ago... before I studied the situation.

The forks extend to the bottom of the hull, they do not go further... and don't dig into the sand. I have filled my sand-trap in a few situations... but those were sandbar issues and it is not fun pushing a flat bottom off a sand bar.

The forks were extended to assist with the problem of cavitation. They immediately helped. The situation re-occurred.. I was experiencing cavitation while trying to get on step. I found it was my 'E' impeller which was beaten to a pulp... see here:

I just pulled my 'B' impeller off this week and polished up the 'dings', normal maintenance for this boat.

I appreciate the comments...notice they were put to use

 

[JetSled]

Re: Modify the old hull?

We completed the modification.... and didn't rotate the hull for this amount of work. It ended up totaling 2 hours.

We found the tunnel portion was only 1.5 inches deep. My buddy Al cut out the 'tunnel' section, and the plate behind the intake (not sure why it was there).

We welded in some square tubing on the sides, then cut and formed the transition, then welded it up. I ground a little, and smoothed out the bumps with some cold weld putty.

I should have the intake grate back on this week (i'm adding a few fins to keep the rocks out). If the hull performs ok, I'll clean it up, round the edges, and finish it off properly .

 

[ondarvr]

Re: Modify the old hull?

It's hard to tell from the pic, but it looks like you have a sharp edge at the front of the tunnel. It should roll smoothly into the tunnel so the water can follow the contour and not create an air pocket.