Want to gain a few knots

I just bought a 1964 23' Uniflite, custom built flybridge design on a former express Cruiser hull. It has an IH 6.9L IDI engine rated at 170HP at 3300RPM, and Twin Disc 1.54:1 trans. The hull has very little deadrise at the transom, and the prop is a 14x17 with only about 1/2 to 3/4 " clearance from the bottom. The boat is likely heavier than production (which were already heavy), and the seller confirmed that its a slow boat. Its a cool custom rig in every way except speed. I bought it anyway even though its slow because of circumstances.

Any off the cuff ideas on what I might consider trying to eek out a couple more knots of speed? A few added info: it has over 200' of chain in the forepeak.. I'm going to keep about 20' of chain and splice on 200' of line instead. That will help a little. It has a 35 fresh water tank mounted under the forward berth. It has 3 fuel tanks for 120 Gal total under the cockpit. I can carry less fuel and minimal water to keep weight down, but I wonder if there is another change to consider. Could a Twin Disc trans be re-geared to a 1:1 if there is enough torque at crusing RPMs to spin the prop? any other ideas?

Thanks.

 

17p seems very low for 170 HP particularly as you have 1.54:1 its the old question ..what rpm does it run at wot .....maybe a change to a 4 blade at greater pitch is the answer

 

That's one thing I've thought - 4 blade for more surface area and catch, but I wonder about getting the prop too out of square by increasing pitch. What I really need probably is larger diameter, but there isn't the room for it. I think first thing I'll try, as long as I can get reach WOT is add cup, then if still able to hit WOT or better, increase pitch a little. If not satisfactory and still able to hit WOT then I need to find a loaner 4-blade to try, or else win the lotto first to buy a new one.

 

in my experience diameter change does not affect performance much ie 13 or 14 in ..pitch is the big factor..... I still dont see 17 p and 170 HP ...a 90 hp outboard will spin that .......well more than than .....

 

How much clearance at the top of the wheel, between the hull and tips?

The best thing you can do is strip the boat of everything that's not physically screwed or glued down. Most boats take on a considerable amount of weight as they age. Remove every spare dock line, all the anchors, everything, then take it for a run and see how much of a difference this makes.

I've seen boats take on tons of extra gear, which can lower preformance expectations considerably. Next find out what the WOT RPM for your engine is supposed to be and see if your engine is pulling within a 100 RPM of that figure. If not, you could stand some improvement in this area. If she is pulling her recommended RPM, then you have to start looking at drag and weight.

 

There is only about 1/2 to 3/4" clearance between the tips and hull. I haven't measured it, just eyeballed it. The prop blades are slightly cut down I'm fairly sure, just by the look of them, so I might have something less than a true 14 x 17 prop. It could be more like a 13 x 17. I'll trial it with gear removed to get a benchmark. First thing is to get the trailer road-ready so I can get the boat to the water. I expect to splash it sometime in December. I'll post back the results.

 

The 6.9 was used in Ford Pickups until the 7.3 came out.
The 6.9 was really heavy. 10 Mpg in a Pickup.

I think you might try to find the Six the Mopar Pickups used. I don't know the numbers but its got to weigh less.

Better fuel mileage, easily the same or better power than the 6.9 and a little better for the Alternator and Water pump life.

 

Sweet God, with so little clearance between the hull and wheel tips, you're screwed. The generally accepted minimum for tip clearance is 15% of the wheel diameter. This means the bare minimum tip clearance should be about 2.1" on a 14" wheel. Naturally, the more clearance the better. I prefer 20%+ of the wheel diameter, which is 2.8" on a 14" prop, as the minimum, but again, more is better (just ask your wife).

 

That's funny... my wife seems to think the exact opposite....

 

hey thud,the 7.3 was a bored out 6.9 with a few small differences and both weighed around 930 pounds.

Cummins 5.9 I agree is a better engine in every way but it's 80 pounds heavier and I think they're 41" long and 38" tall so it may not fit.

 

i don't think you can do a lot with that shaft setup. why don't you look at a sterndrive or maybe a prop pocket.

 

You could perhaps lengthen the shaft. What is your shaft angle and maybe show a picture from the side? Clearance to rudder?
Sounds like someone got a free prop and made it fit when they re-powered the boat. Yo could definitely do better.
You could always do better if you wanted to re-power again, but I don't think it would be worth the money. Online I even noticed an outboard on one?

 

I would never have believed the 5.9 was 80 Lbs heavier!!!
I had both and a Mitsubishi too.
The 5.9 was a bit slower on long hills under a load, but it never had the Perforated Cylinder walls either.

The fault with all Diesels is the Vibration which kills the Ancillary stuff, like Pwr Steering pumps, water pumps, Alternator bearings and the Air Conditioning pumps.

 

The specs I've found on the 6.9L state that it weighs approx. 860 lbs. The guy that built up this boat put new strut and shaft log in when he re-powered it. I don't know exactly why he didn't create more clearance, but maybe the down angle of the trans and the way he bedded the engine just didn't allow it. The shaft can't be lengthened to any degree due to the rudder. In every other way, the work he did on this rig is top notch. He was a Uniflite dealer for a period of time in the 70s and spend his whole life on boats and around the water, so he knows boats. Unfortunately, his memory has gotten a little foggy (hence part of the reason he had to sell among other ailments), so I can't get all the particulars from him about decisions he made at the time of installation. I don't intend to repower the boat - its just not worth the time and cost with the boat market so dead. I just want to get the maximum potential out of the setup that it in there. Thanks for all the feedback. First thing will be sea trial to test WOT, then decide what to try in changing pitch and/or cup on the prop. The only other thing I've thought about, mentioned in a post above, was the idea of creating a small prop pocket or tunnel. That's a bit of work, but could be done without impacting the whole installation. I did once see a 27' Uniflite Express Cruiser for sale that had been converted to diesel and the guy had created a tunnel that apparently worked well. Lastly, I did consult my wife about the situation, and she seems to think that a longer shaft and a little more diameter could provide more thrust....

 

Sounds like your wife know what mods she would like to have made.

A tunnel sounds like a lot of work to me. Large potential to fail too buy having poor flow into the prop and decreasing hull lift at the stern (gives me the shivers).
You would likely then have to change the p bracket/strut in any case.
That just leaves the changing the angle on log the or lowering it and depending on your drive line maybe a slight engine lift and tilt (or universal) to change shaft angle?

If you want to try another prop first it should probably be closer to 12" diam 5 blade.