Cavitation plate height?

[valvebounce]

I have a 10ft f/glass rowing boat,I intend using an Evinrude 6hp with a 16"leg on it.From the transom to the lowest part of the hull jt measures 16".
Beyond the hull there is a 4" external stringer/keel,plus two stringer stabilisers either side.
The boat is quite stable as you can imagine.
Would it be ok to use the motor at this height,or would the turbulence from the keel interfere with the performance?
The transom is sound,but I am raising the height either side of the motor to cope with swamping when It is launched,plus on occasion a following sea maybe.
The boat needs to be trailer launched because of the weight.
Any advice will be gratefully received.
"V"

 

[lewisboats]

First off... motors come in 15, 20 and so on lengths. If the hull is 16" from the top of the transom to the bottom of the hull then, ideally, you should have a 20" motor. The plate should be at or below the bottom of the hull. The keel may induce some turbulence to the prop but I expect it won't be that much of a problem. A 20" leg on the motor should be perfect. Does the transom have any rake to it or is it plumb. If it is plumb then you might consider making a motor mount with some rake to it. Most modern (post 1930's) motors are designed to fit transoms with a 13 deg rake aft. If you put it on a plumb transom the prop will automatically trimmed upwards, driving the transom downwards and adding more bow lift to a hull presumably already not designed for a motor.

 

[PAR]

Straight line performance will suffer a little from the skeg, but turning performance will likely experience some ventilation issues from the wash off the one side of the skeg.

Can you post a picture of this boat, so we can see the run and these skegs?

 

[valvebounce]

First off... motors come in 15, 20 and so on lengths. If the hull is 16" from the top of the transom to the bottom of the hull then, ideally, you should have a 20" motor. The plate should be at or below the bottom of the hull. The keel may induce some turbulence to the prop but I expect it won't be that much of a problem. A 20" leg on the motor should be perfect. Does the transom have any rake to it or is it plumb. If it is plumb then you might consider making a motor mount with some rake to it. Most modern (post 1930's) motors are designed to fit transoms with a 13 deg rake aft. If you put it on a plumb transom the prop will automatically trimmed upwards, driving the transom downwards and adding more bow lift to a hull presumably already not designed for a motor.

Many thanks for your reply,
The transom has a slight rake to it, about 10%.The transom has a slight external bow in it,about 3/4".

 

[valvebounce]

Straight line performance will suffer a little from the skeg, but turning performance will likely experience some ventilation issues from the wash off the one side of the skeg.

Can you post a picture of this boat, so we can see the run and these skegs?

Hi PAR,thanks for your reply.
I'll post a few pics.
The boat hasn't been in the water yet,I picked it up off ebay.The boat was sound,but had a few half hearted paint jobs on it,and had been out in the weather for a while.I flatted out the hull and painted it with two part epoxy paint,then overpainted it with white garage floor paint.
The transom has a checker plate bolted on it to take a motor.I have fitted new oarlocks and have new aluminium oars for it.
I picked up the 6hp evinrude so I can use it for close inshore fishing.

 

[valvebounce]

Some pics

 

[valvebounce]

Put some pics of the boat on,had to press "cavitation plate height to get them up.

 

[PAR]

How fast are you hoping to go, as the buttock angles seem pretty steep for anything more then low teens (MPH). The centerline skeg will be chewing a lot of water to the prop, so it'll likely vent pretty badly, especially as speeds pick up. In fact, it looks like a sailboat hull, because the keel swells near midship, where a centerboard would penetrate. Simply put, not well suited to going very fast.

 

[valvebounce]

I'm not really in it for speed,I was hoping for a bit of lowdown torque so as to make way in currents etc.
From what you have said it sounds like most of the power would be wasted.
I've got an Evinrude 18hp fast twin,which has a 19" leg,but I don't want to overpower the boat.
I've not had it in the water yet,but with the heavy keel I imagine it will be very stable for a bit of fishing.
Do you think doel fins would help,or would a longer leg be better?

 

[PAR]

Unless the keel is significantly weighted (not likely), then no additional stability will be provided by the slight protuberance on the bottom centerline.

I think she's a displacement boat and any more power tossed at her will just waste fuel for very little gain. The 18 HP outboard is way more then she can handle.

 

[lewisboats]

Wow! that is some seriously thick stuff for such a little boat. Looks like it could be dragged over lava beds with little damage. Yeah... those are gonna cause some turbulence. Does it look like there was a dagger board or center board slot in there ever? That looks way too over built for a purpose designed oar boat... but perhaps a tender in an area with coral?

 

[valvebounce]

Unless the keel is significantly weighted (not likely), then no additional stability will be provided by the slight protuberance on the bottom centerline.

I think she's a displacement boat and any more power tossed at her will just waste fuel for very little gain. The 18 HP outboard is way more then she can handle.

Thanks PAR,
I'll just have to give it a go with the 6hp.I imagine it will row pretty well.
I can't figure out what the purpose of the boat was initially.it does seem to be a bit of a "One off"
When I turn it over from upside down to the right way up,there is no stopping it after the point of balance,it just self rights no matter what.
My brother seems to think it is a funfair boat from a leisure park.

 

[valvebounce]

Wow! that is some seriously thick stuff for such a little boat. Looks like it could be dragged over lava beds with little damage. Yeah... those are gonna cause some turbulence. Does it look like there was a dagger board or center board slot in there ever? That looks way too over built for a purpose designed oar boat... but perhaps a tender in an area with coral?

Thanks for your reply,
There is no evidence of it ever having a dagger board or sail.
The channel above the keel is only about 2" deep on the inside of the boat,so the rest of the keel must be solid.
Funfair boat maybe?