Prop Pocket

Hi Guys,

I have a 26 foot half cabin with an 8 foot beam and 16 degree transom deadrise. The hull was originally an inboard outboard which i am converting to a mid mounted inboard. The engine is a 220hp turbo diesel and full loaded i expect the boat to be 5500pounds in the water.

Due to trailering and draft considerations, i am incorporating a prop pocket into the hull of approximately 3.5 foot of straight half circle out from the transom and then tapering back a further 3 feet amidships which then feathers into the hull deadrise.

From my research i was recommended reading an article named 'farther faster and more fuel efficient' by Dudley Dawson in the Professional Boat Builder Magazine. I came up with a design based on his guide for tunnel design for small craft;


The key points he mentions are:

1) All tunnel surfaces sould be as smooth and fair as possible without knuckles.
2) the tunnel should be as wide at the forward end as it is at the transom
3) At the forward end the tunnel should fair into the boats profile using a gentle s curve shape
4) Tip clearance of 5-10% of prop diameter will be achieved.




So for some indicative dimensions:
Tunnel radius at transom is 12" and the tunnel height above the original keel line is 7".

I'd really appreciate any comments on this design and in particular if i have done anything majorly wrong here.

Looking forward to some feedback.

Nick

 

"3.5 foot of straight half circle out from the transom and then tapering back a further 3 feet amidships"
I presume you mean 3.5 feet straight section forward of the transom, and tapering out over another 3 feet forward.

I would personally do the CAD drawing starting from a surface, not sections. Once you have a smooth surface, then draw in the required structure. You'll need to match any existing bulkheads (photos and dimensioned sketches are useful), and also provide adequate support for the prop-shaft/stern tube. Pay particular attention to this, as the whole drive train may be under water, and it's probably the most likely thing to leak!

Having got the design sorted, I would build a male plug (outside) of the tunnel, polish it, then mould the tunnel over the plug, laying in structure where needed, using a jig attached to the mould.

This is not an area that I am particularly familiar with, but your suggestion looks sensible.

Cheers,

Tim B.

 

This looks like a disaster waiting to happen to me. I don't think this will work too well at the speeds that you expect from 220 Hp. You need to get a serious computer program to figure that shape at speed. No way 7" recess. Maybe 4" recess at speed. What you show would work at about 7 knots okay.

Think about the curvature of the the wake coming off your transom at speed. That is about the curve you can work with for the pocket. The other option is a square front pocket that is fully ventilated. That is also a bit of black magic. I don't recommend that you do this unless you can get an experienced go fast guy on board and ride on a few of his boats. Living with a smaller diameter prop would be better than this cobble. Putting the I/O back in would be better still.

 

Shape of pocket:
- 1:7 taper (depth to length)
- Intake area should not be circular, it should be elliptical coming to flat
- Circular section at prop
- Height of pocket should be max at prop section, can be reduced further aft and fwd
- Aft section of pocket might have flat top (for better lift and rudder arrangement)

There is excellent paper by D.Blount on pocket design...

 

Thanks for the feedback guys:

PhilSweet - Sorry I should have clarified – the pocket sides are actually 4.5inches but the height from the original keel line is 7inches. From many of the pockets I have researched, this is a standard depth for installations of this nature and size.

Alik – In response to your items I have provided comments against these:

- 1:7 taper (depth to length) – THIS WILL BE ACHIEVED

- Intake area should not be circular, it should be elliptical coming to flat – THERE ARE NO CIRCULAR SECTIONS, THEY ARE ALL ELIPTICAL USING A 24” DIAMETER RADIUS

- Circular section at prop – SEE ABOVE. IT’S A 24” RADIUS.

- Height of pocket should be max at prop section, can be reduced further aft and fwd – DESIGN HAS THE POCKET HEIGHT CONSTANT FROM THE TRANSOM TO 1 PROP DIAMETER FORWARD OF THE PROP. IT THEN TAPERS AND REDUCES IN HEIGHT OVER A 1:7 RATIO. MY CONCERN WAS PRIMARILY WITH THE SHAPE OF THE TAPERED SECTION.

- Aft section of pocket might have flat top (for better lift and rudder arrangement) – I AM GOING TO BUILD IN FLAT REBATES SO FLANGE OF STRUT IS RECESSED WITH THE POCKET TOP TO REDUCE DRAG.

Looking forward to more feedback guys.

Does anyone know where I can get a copy of the d.blount paper?

 

Rework of design based on the above comments....





Any thoughts?

 

Have a look at this for an idea

http://www.boatdesign.net/gallery/showphoto.php/photo/15817/size/big/cat//ppuser/765

 

this is a 24 ft guardian i converted to an inboard from a sterndrive . i fitted a detroit 453 turbo , 165 hp. the boat performed really well . except reverse was hopeless with out the paddle wheel effect. cruise was 17 knots at 11 lt an hour. top speed was 22.5 knots. if i was doing it again i would fit the diesel and run a jackshaft to a leg. the prop pocket was a good project but a hell of a lot of work.

 

i also had to use the trim tabs a lot more because of the loss of bouyancy in the transom.

 

Tunnel not a pocket

There a lot more to it than just cutting a hole and poking the prop in it and hoping for the best !!. The feed to the propeller needs to be long and smooth and a gental flowing shape with no shape corners to cause turbulance and edies with air pockets !! the aft end also needs to be well thought through , the clearance of the prop to the top , and sides of the tunnel and the number of blades and the strut a single leg ?or a twin leg? and the angle of the legs if they are in time with the prop blades can cause harmonic vibrate !! .Lucky you asked !,cause theres so many things to consider and be aware of !!
Had a boat builder friend that got taken to court and cost him $4000;00 because he listened to some idiots in a propeller company that didnt know what the hell they were talking about !!
Read all that will get posted and see who knows what they are talking about before you make up your mind as to what you are going to do !!.

 

Tunnel Stern Design

John and Wiliam Atkin designed some successful tunnel-stern designs like this:http://www.atkinboatplans.com/Cruisers/RiverBelle.html

 

Is it not advisable that the junction of the half cirle with the bottom also be well radiused, rather than a sharp edge ? My question is what additional prop efficiency losses accrue due to the closer proximity of the prop to the hull, as in the prop working in entrained water more than a normal set-up ?

 

What are the reasons we want a tunnel ??

first thing that comes to mind is the draught of the boat will be reduced !
Second its is possible to creat a semi jet effect and almost a jet of water coming out the back !
By installing twim rudders the turning becomes much more positve than a normal set up . The lead into the propeller needs to be as gentle and free flowing as possible !!,sharp angles and pockets will create turbulance into the propeller !!,a flow of water lifting into the tunnel will have a powerful sucking effect but if the tunnel is long with a gradual entry then this will be dramaticly reduced and again more efficent . The closeness of the multi bladed prop to the tube again is an efficentcy issue!, part of the tunnel is a semi jet effect so theres a mixture of differant elements come into play .The big super yacht i worked on had two 3000 hp motors and twin tunnels and i was totally in favor of the way it was set up and put together !Long ! Fair !, smooth and had a slight gradual turn down from where the propellor was to the aft end

 

Its neat but would like to see the prop further up inside the tunnel !!!

 

The forward part is to abrupt and needs to be smoothed out heaps Check out the other drawing !! they have it right !!.