Best prop for pocket tunnel

What would be the best prop for a pocket tunnel? 4 or 5 blade?

 

Hard to say simplifying it down to that, but the 5 blade is probably better able to cope with a less than ideal flow into it, but likely loses a few revs, by comparison.

 

I'm really looking for a good hole shot in skinny water. Anyway, this is my design.
The tunnel is 6 feet in length x 6 inches of rise x 14 inches wide. I plan to vent it with some thru hull fittings. Not quite sure how it will work, but, I want the most efficient prop I can get for the job. It will be powered with a 70 hp Johnson. All I can do is try.

 

You will get hole-shot out of a five-blader, but top-end speed is reduced, by the extra blade area.

 

Ten four. I'm not really worried about speed. If I can get 30 - 40 mph it will be fine. I'm just worried about cavitation.

 

A 70HP Johnson has a gear ratio better suited to lower speed applications, you'd be hard pressed to cruise at better than 30 mph, I would think, but depending on other factors like how heavy the boat becomes. Ventilation can be adjusted for with motor height to some extent, so install the engine in such a way that the mounting bracket holes allow you to adjust up and down. The higher you can get away with, the better for speed. Of course I assume all props to be used and tested, will be stainless, alloy props are far more inclined to cavitate and ventilate, because the blades are thicker.

 

I plan to use stainless props. The 70 I have is an old 2 stroke. 1980 ish model. I have to go through it, I plan to push the envelope a little and see if I can make it a stinger. Those stingers were some hot motors. I dunno, it depends on how much it will cost me for the parts.

 

First welcome to the forum and I see you're pretty much right in the path of all this weather we're hearing about. Good luck and stay safe.

As for your prop, there are many variables. Generally four or even five bladed props are used in situations where prop to hull clearance is restricted. That's not an issue for you based on what I can see. Four or five blades can solve problems with trying to push too much horsepower through too small a propeller. You don't have that issue either. A four blade will probably get you on plane a little more quickly but more importantly keep you on plane at a slower speed. That might be beneficial. Not really necessary though. So a three bladed prop might work just fine but you need to run the numbers.

If you want to do it yourself and are willing and able to do some high school math , I'll post a link below to a book that you will find very helpful, but only if you take the time to understand it. Otherwise head to a good prop shop and show them what you have there. With all the fishing and shrimping going on near you there must be quite a few folks with good reputations that you can find.

Good luck,
MIA

Propeller Handbook: The Complete... by Dave Gerr https://www.thriftbooks.com/w/propeller-handbook-the-complete-reference-for-choosing-installing-and-understanding-boat-propellers_dave-gerr/496232/item/5282955/

 

Thanks for the welcome. I've been out of boating for so long I'll have to find a good prop shop. These pocket tunnels have one issue, and that's the lack of vacuum without a proper vent causing cavitation. I'm thinking if I go with a smaller diameter/more pitch prop it may cancel a lot of the cavitation problems. It's trial and error. I will just have to do some research and trials to find the best option.

 

I'd say smaller diameter props with more pitch are more prone to resent a lack of "clean" water. The OMC 70 swung quite a big prop diameter, anyway.

 

After rereading my last post, I made a terrible error. It should be bigger diameter, less pitch.

 

Well, maybe an error, but not a "terrible" one anyway.

 

Lol, thanks for the vote of confidence.

 

I expect that you are going to have issues with the tunnel ventilation on any more than a mild turn.
We have built many outboard jet with tunnels and there can be issues with the tunnel ventilating.

An assumption, there are not any directional strakes on your hull, ie the boat will slide in a turn. Especially with a 0 degree dead rise hull

Picture this
Moving straight ahead, the tunnel fills with water and the prop is grabbing blue water, ie no air.
The transom is ventilated and the tunnel is full

Then you turn, left for this discussion, quickly, an avoidance turn, or even a moderate turn, the boat begins to slip. Viewed from the rear, the left hand vertical edge of the tunnel becomes a transom and it will ventilate right down
to the level of the water leaving the rear transom edge, leaving the prop without water within the tunnel.

We had this happen with some of our first designs with about a 3 1/2 inch tunnel. The upward tilt to an outboard jet intake has a vertical cross section area of say 6 inches wide by 2 inches in height.
Even with such a small vertical height tunnel the tunnel ventilated and the jet slipped.

So we made some changes
1) tapered the vertical edges of the tunnel to the bottom of the hull, this allowed the water level in the tunnel to rise quickly to keep the jet intake flooded a bit better
We also radiused the bottom to vertical tapered edge.
2) increased the width of the tunnel, as above, a wider tunnel lets the water level rise a little higher
3) shortened the vertical height to about 2 1/2 inches, we found that with the smaller height that the tunnel did not ventilate as quickly
4) installed directional strakes, 3 per side on a 5 - 6 foot chine width, to minimize the boat from sliding.

Each change improved keeping the jet intake hooked up through harder turns.

When I look at the back of your boat, flat bottom, vertically deep tunnel, no directional strakes, I suspect on trial day that you may find that on a tighter turn that the prop will lose traction due to ventilation of the transom.

We never did hang an outboard with a prop off the back though, and perhaps then can handle a good percentage of the prop out of water. There will be experienced people on this forum who might be able to make suggestions
of partial surface piercing props which might work quite well with flow over only part of the prop diameter

 

It is not clear if your hull is a monohull or catamaran.

Is this the tunnel you have created in your monohull:


or, is this the gap between the 2 hulls of a catamaran?