Need to gain 2-3 knots

Here goes - picked up a boat a couple years ago -couldn't turn it down for several reasons, partly because the owner was a great old guy and wanted a good home for his boat. Its a cool boat, but very slow and doesn't quite get on plane. Runs at about 12 kts, and I can 'feel' that it would plane at about 14kt or above. I've used enough different prop/ power models and other calcs that make me think it should be achievable, but power/ gearing/ prop setup is less than ideal.

Basically, I'm just looking for informed opinions whether or not my goal seems reasonable to achieve with some modification, or if I'm just wasting my time.

Here are the specs:

23' hull, very low deadrise at stern (guessing 10 degrees). Uniflite Hull with custom structure.

Converted by former owner to single diesel, 6.9L Pleasurecraft Marine conversion, rated 170HP at 3300RPM.

1.54:1 Twin Disc gear, and 1-1/2" prop shaft.

14 x 17 prop (according to stamp) 3-blade. I think the prop was cut down from larger diameter, because blade curve looks slightly odd.

Cannot accommodate larger diameter - not enough clearance to hull.

Total displacement, loaded estimated at between 7,500 to 8,000 lbs.

Currently, at 2,900 RPM, I only get 11-12 kts. that calcs to roughly 60% slip if prop actually is a 17 pitch - very horrible.

In theory, this engine has enough torque to turn more pitch, but I recognize that hull design, weight, angle all affect the drag and amount of power required.

I just saw an ad for a 29' Dyer with 150HP single diesel inboard that reportedly makes 17 kts running. I can't think that a 29' flybridge boat is any lighter than my 23' boat, and I don't know if hull shape alone could be that much more efficient. This give me some hope that there is room for improvement in mine.

The ideal prop according to calculators is a 16 x 16 at the 1.5:1 gearing, but Ican't do that. The calculators also suggest I could get max speed of over 20kts. Same calculators indicate that with a 14" diameter blade I should be using a 19-21" pitch, depending on the displacement assumption I use. I can't change gearing, can't change shaft angle or prop clearance, and can't increase RPMs without major expense that I could never justify, and I can't meaningfully lighten the boat. Pitch change is the only logical place I can think of to make a difference. BTW, I don't think any production blades for 1-1/2" shaft even come as small as 14" diameter, which is also why I believe the current blade was cut down from a large diameter.

Any opinions? Does it sound like it would be worth the effort to try re-pitching the prop higher? Would lifting rails added along the chine possibly be worth considering as well if hull is overweighted for its current design? Look forward to any thoughts or input.

 

Are you really shure that the propeller is 14" diameter and 17" pitch (This is the meaning of the writing 14x17)? I'm asking, because a 17" dia x 14" pitch would have been the logical choice for this boat and engine (16x16 would not be the best choice btw) .

Also, check the gearing; could it be that the gear ratio in fwd and reverse are very much different, and the prop rotation has been changed.

If, indeed, your prop is 14" dia, and neither diameter or gearing may be changed, then your only option would be a nozzle, since the propeller is extremely heavily loaded.

Given the wheight and overall dimensions stated, 18 to 20 knots should be quite normal, provided a relevant prop dia was used.

 

Baeckmo,

Thanks for the reply. I'm very sure the prop is only 14" diameter by 17 pitch, and not the other way around. I never thought to double check the shaft speed to insure that it does in fact turn at the stated gearing in forward gear - it is worth a check a tach gun since that is an easy thing to do. Thanks for that idea.

I'm not quite sure what you mean by a nozzle however - can you clarify?

Thanks!

 

He means a shroud around the propeller, to accelerate the flow into it.

Here's a picture of a large propeller in a nozzle:

 

I see. Doing a very quick google search, I came across a definition/ discussion of a Kort nozzle and Rice nozzle, plus some old threads here. I quickly get the sense that the nozzle use is more effective at increasing thrust at slower speeds generally under 10 kts, and that the drag from the nozzle begins to outweigh the added thrust at higher speeds. If this is correct and my goal is to gain a few knots in speed, would the nozzle concept actually do the opposite? I appreciate all the comments here, because I'm certainly not an engineer, just a layman trying to be able to make improvements with limited options, limited knowledge and a limited budget!

 

Tailshafts are used on large vessels, because when the shaft is pulled for required inspection, they don't want to pull and lathe 50 ft of prop shaft. So a short tail shaft is coupled to main shaft and prop fits on the tail shaft. Only the tail shaft gets inspected.

I mention in order to ask if there would be room to extend your shaft and gain an inch or two of clearance for a larger prop?
if there is room, then a custom tailshaft and coupling could be turned in on a lathe.

Or install a longer shaft.

 

Another interesting idea, but I wouldn't have enough clearance to the rudder to simply lengthen the shaft to increase the effective clearance to bottom of hull. I had thought about this idea if I could easily move the rudder farther aft, but I could only effectively move the rudder aft by 2-3 inches at most, and I don't think the increase would only gain less than 3/4" clearance, or added prop diameter capacity of only 1" to 1-/2". I suppose some is better than none, so I will re-look at this option a little more closely as well. Thanks.

Although not a prop question, on this same issue does anyone think that adding lifting rails at the chine might also provide better lift, and therefore produce added speed? I think the boat is heavier than the original production boat.

 

can you move any weight forward? add some spray deflectors at the stem with sufficient angle to get enough lift to offset the weight you moved forward.
getting your stern up a bit and getting your prop shaft nearer level so prop thrusts directly astern, these might help and cost little or nothing.
If your prop isn't turning top rpm then try a two blade, high pitch, speed prop.

have you considered mounting rudder aft of transom?
Something like an outboard motor mount but hang your rudder on it. You might get your propeller aft of transom also then it could be huge. but would need a guard over it.

 

Based on all the comments above and on reading some other threads, I have come up with a list of ideas to try.
1. Get prop looked at and maybe add a little pitch
2. Consider adding lifting rails along chine for and aft. I think the boat doesn't have quite enough planing area for the extra weight versus original design.
3. Consider extending the hull by 1-2'. Remove the swim step, and make a molded step/ hull extension to increase total area. This could also allow rudder to be moved aft much farther and then allow a longer shaft (and new strut) to accommodate a 16-17 dia prop.

4. Quit worrying about it, sell or give away the darn thing and just get a boat that already does what I want it to do!

 

You seem to have two main issues here;

A/ The weight is high for the waterline length, resulting in a high resistance hump in the 10 - 14 knot region.

B/ The propeller is far too small in diameter. The resulting thrust per blade area unit is very high. To cope with the engine power, it has an unhealthy high pitch. With the figures at hand, it is clear that the propeller is cavitating heavily, leading to a serious thrust reduction.

The thrust versus speed for your propeller is very flat (ie low at low speeds and relatively high at high speeds), compared with the more suitable 17x14. The thrust available in the hump region is not enough to take you through to higher speeds.

You have to adress both issues in order to get a functioning boat. Start with the following:

A/ Check everything onboard from stem to stern; remove every item that is not necessary, down to the forgotten "nice-to-have-in-case" nuts and bolts hidden in a locker. You will be surprized by the weight of the stuff added over the years.

B/ Add bottom surface in the shape of 0,5 m tabs over the full transom width. No angle of attack, just continue the existing bottom shape. NOTE: make them strong, with good brackets.

C/ Get a new propeller with as much blade area as you can get, say 4 blades, blade area ratio minimum 0,75. It must have LESS pitch than you have now, say 15", but with a heavy cup. Every possible extra millimeter of diameter is a bonus, provided a reasonable clearance (10 % of dia) can be achieved.

In your previous note you say that you intend to increase pitch. That´s the worst thing to do with a cavitating prop. Instead the leading edge incidence must be reduced and the lift must be created by increased loading near the blade trailing edge (ie cupping).

Adding rails will increase the resistance in this speed region, so don't.

Would be fine to see some pics of the bottom and propeller/rudder arrangement. Btw regarding nozzles: there is far more to the subject than you will learn during an Googling evening; it might well provide a solution in your case.

I think this boat deserves some decent care and a good home, you've got the chance, take it!

 

Thanks to everyone for all for the advice! This last post is especially helpful and informative. This boat is very unique, and other than the slow speed it is a one-of-a-kind pocket sportfisher. It is worth trying to fix this issue. The former owner built it up from a bare hull into the current boat. I restored it from a deteriorated condition after the former owner let it sit for years after becoming sick and disabled, and then reconfigured some of it as well, so a lot of care has gone into it during its life and rebirth. I'm encouraged that there are potential solutions to pursue. I don't have any photos of the underside of the hull to post, but here are some photos of the boat in general, just so you know what it is.

 

a beauty

 

I would agree with what Baeckmo says.
some brief comments:
-A quick speed estimate gives me about 20 knots max.
-As to the prop diameter, IF possible (hp, rpm, tip speed, etc) go to about 18" dia. minimum to get decent blade area.
-I think a contributing factor to the weight issue could be that flybridge.... it looks quite massive and probably has lots of 'storage' to hold stuff (ie weight).

 

I think all the original mods by prior owner added weight - lots of 3/4" ply throughout for decks, floors, lockers, and yes, flybridge. Total weight on trailer was just 8,300 lbs so maybe 7,300 for boat with 1/2 full tanks, no people. Hull extension makes logical sense to me, since to my layman brain added weight over original design requires added hull surface area to compensate. I also think Baeckmo nailed it about cavitation - after about 2400 RPM or so up to 2900, no added speed, and gives the sensation of 'slipping', which after reading Baeckmo's comment I am now nearly certain that the prop is experiencing cavitation, kind of like the prop is saying "I give up" through the last few hundred RPM. Larger diameter prop has got to be the key, and the only way I see to do this is to add a hull extension and move the rudder aft to allow a longer shaft (and of course new strut) and then install 16-18" prop, depending on resulting clearance. I'm trying to figure out a way to test the hull extension first with temporary plates of some kind before moving to a bolt-on extension/ swim step type box. Maybe just flat aluminum plates secured to the trim tabs with extra bracing all along, and insure that it doesn't create an edge that sticks down below the existing hull that would create drag?

 

>Larger diameter prop has got to be the key, and the only way I see to do this is to add a hull extension and move the rudder aft to allow a longer shaft (and of course new strut) and then install 16-18" prop,<

While its hardly the best solution a steeper shaft angle might cost far less to get the diameter you need.

IF you can work with it I have a Michigan Right hand 5 blade 20 diameter 16 pitch 1 -1/4 shaft to loan or sell.