XF20 transum hieght

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L B Lee
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XF20 transum hieght

Post by L B Lee » Wed May 09, 2007 7:10 am

I am building a xf20 and need to no how high to make the transum. I plan on using a cnc jackplate and a 20" shaft engine. I dont have either right now so I'm stumped. please advise :doh:



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jacquesmm
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Post by jacquesmm » Wed May 09, 2007 10:14 am

The plans show the transom height but I understand your concern because some jackplates have strange mounting bolts patterns. Most jackplates, 99%, will fit on the transom as designed but it is a good idea to buy the jackplate at this stage and check. No need for the motor.

I have seen weird things with jackplates. When Ken O. built his PH15, I had the dimensions for the jackplate but they refused to give me the engineering drawings. It fitted almost: the lifting part was a 1/4" wider than their dimensions but worse, there was grease nipple where we couldn't reach.
Ken found a solution but iIf you have the jackplate from the start you can avoid those problems.
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gpratt
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Post by gpratt » Wed May 09, 2007 12:52 pm

Make sure that you have 20" from the top of the transom to the top of the tunnel. I'm not sure what the current plans specs are but I had to add 4" to the top of the transom. I think the original plans had 20" from the transom top to the bottom of the hull leaving 16" from the transom top to the top of the tunnel. With only 16" between tunnel and transom top many jackplates will be sticking down into the tunnel area and will be in the flow coming out.

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jacquesmm
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Post by jacquesmm » Wed May 09, 2007 1:36 pm

The plans are still the same. It really depends on the jackplate. I remember that you have a heavy duty one.
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ponyboy
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Post by ponyboy » Wed May 09, 2007 2:53 pm

Another good reason to have the jackplate handy, Did a "re-mount' of an E-TEC last summer, originally did not come with jackplate, we wanted to add one.
Problem: jackplate fit transom, however the lower mounting holes put the mounting bolts right on the floor of the boat. Took an act of God and congress to figure out how to remedy the problem.

Jacques and Ken are right on and prolly know more than I ever will, just food for thought from personal experience.
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L B Lee
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XF20 transum hieght

Post by L B Lee » Wed May 09, 2007 6:17 pm

Thanks guys for the fast reply.
Last edited by L B Lee on Wed May 09, 2007 6:21 pm, edited 1 time in total.

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Re: XF20 transum hieght

Post by Hunch » Wed May 09, 2007 6:17 pm

L B Lee wrote:I am building a xf20 and need to no how high to make the transum. I plan on using a cnc jackplate and a 20" shaft engine. I dont have either right now so I'm stumped. please advise :doh:
IMHO adding the extra 4" to the original plans make the lines of the transom ugly as snot. Besides the transom sticking above the rear deck (if you are building the decked version) that extra hieght will make boarding after taking a refreshing swim a real pain in the ass.

The hole shot with the high transom even with the CNC all the way down leaves alot to be desired. The cavitation plate is just too high.

The cavitation plate should be no higher than the top of the tunnel with the jackplate all the way down.

I built my transom at 15" above the tunnel top or 20" above the bottom of the boat. With careful positioning of the oblong motor holes and jackplate holes you can get the upper and lower ranges just right. My hole shot with a 4 blade prop is very good and I can run across most any flat without worrying about starving the water pickup.

To make hole location for drilling the transom easier hang your motor on a lift. Bolt the jackplate to the motor and raise and lower the chains to get the height where you want it.

Pratt had to add the 4" because he uses a Detwiller jackplate which is not designed for tunnel hulls and has a lower crossmember that drags across the tunnel outflow. No such problem with a CNC.

Good luck

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gpratt
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Post by gpratt » Wed May 09, 2007 11:49 pm

No Pratt didn't have to make it 4" higher because of a jackplate. I'd already made that decision before I had ever concidered which jackplate I was going to use. I did because the tunnel top was going to be the water level that my engine was going to be operating at so it made sense to ti build accordingly. I always had the optiion in mind to use a jackplate to lower the engine down further below the top of the if needed.

I ran the boat with the motor bolted directly to the transom before installing the jackplate for the first time

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Post by Chris Sopotnick » Thu May 17, 2007 10:14 pm

If you haven't purchased one yet, consider Bob's Marine 4" ultra jack plate. I just installed one on the xf 20 I'm building with a 60 hp suzuki 4-stroke. This plate measures 13.5" tall. I have 18" from top of tunnel to transom and left apprx. 1/2" at top and 4" above tunnel. I have not run it yet so I can't give you any pointers on how it rides with this plate, but it was easy to install and is relatively light weight wiht the pump located in the boat. If anyone has had runnning experience with this model let me know.

-Spot
spot

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Post by Ankle Deep » Fri May 18, 2007 2:13 am

Hi Chris:

I too was considering Bobs's ultra jack plate. I like the weight and I recall that the XF20 was designed with the Bob's style jack plate in mind. My only concern is the set back. I think a 6" setback is specified for this boat. Do you plan to use spacers to get to 6" or will you go with the 4" set back?

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