The mystery of a proper prop and terrible performance

What is the gross weight as tested ? Just sounds to me as though it is too heavy for the available power. Edit: OK, I just looked back and the weight was given as 7000+ lbs. I don't know what sort of deadrise this boat has, but it should be something akin to a 25 foot Bertram size and weight wise, and that is a deep vee. Would a 302 CID put a Bertram up on plane. Well yes, but it would be no speed machine, I just can't get my head around a prop of that diameter and pitch , gear ratio 2.05 to 1, 3000 rpm not getting to 10 knots.

 

I've never seen a 25 Bertie with a single 302...but I've seen them with twin 302s. I'm not saying they don't exist or anything, I'm just saying what I've personally seen.

All of the outboard conversions I've seen have ended up with twin 200s, minimum. I saw one with twin 115s once but the buyer found it unworkable and immediately repowered, I think with 250s. He's local, I can check.

Anyway I think you could get one to plane on a 302 if you did everything right, got a short haircut and skipped breakfast, but you sure wouldn't have much breathing room.

But I bet if you built a 302 with all the trimmings and revved it near its limit, it'd go. My old PCM 302 has a 4200 redline, and pushing her up around 4000 on a well-propped 25 moppie etc would probably get her past the hump and you could back down to 3500 and still move near 20 knots, maybe. You'd sure have to have everything right, though. My boat is half the size of a 25 Bertram and has a tiny bit less deadrise and man...you never wish you had less power, that's for sure. I cruise her at 20-22 knots. If I were on calm lakes I think I could get away with a more aggressive prop and probably cruise a little faster, but on rougher water I find I need a "torque cushion" to put a low planing speed right in the small engine's power band, so I run a 17 pitch prop.

Which is mated to a 1.61:1 leg, incidentally...so I'm seeing prop slip up around 25%, just ballparking the speeds and cruising RPMs I'm used to using.

Anyway at 2.05 gears, and a much heavier boat, even with a more efficient prop I have a hard time imagining easy planing with more pitch. But it should be possible to get one well into the teens at least: a quick glance around the internet shows lots of them running with 300hp 350s. So a well built 302 should have a chance, I think.

 

I'll be playing around with her over the coming weeks. I can probably strip out about 10% of her displacement if I have to. That's not a permanent solution, it's just for some experimental runs. It could be wishful thinking but she feels to me like she's close to getting over the hump. The 302 is freshly tuned and running great. No noticeable vibration in the drive line. If I can get her closer to planing off or even on a wallowing slow plane then I'll know that a set of tabs will give her the little extra lift that she needs. I would be very happy with 20 knots. I'm not looking for speed, I'd just like to be able to move on the occasions where I really need to.

Like I said, we'll just have to see.

 

A 6-71 blower might get her there.

 

There are a few Bertram 25's around with twin 130hp sterndrives, a 5 litre V8 would plane one. This boat may be somewhat heavier though.

 

I have seen them with single diesels around 200hp, and that is a heavy engine

 

I've always had a back up plan. The way I see it a set of 24" x 12" trim tabs would be the easiest option. I'll try that first, next spring. If that doesn't solve the problem there is, as you and Mr E mentioned, another option. Just about everything on my 302 (except the intake manifold and distributor) will swap over to a 351. The new v-drive can take the additional power and the new cooling system is rated for a 350 CID engine. A 351 swap would probably run in the neighborhood of $4,000 and some work on my part but not too much really. I'd much rather keep the 302 in there, so for now the trim tabs are the best option. Still love my little boat!

MIA

 

I wish you well, but at 10 knots, they don't do that much, the tabs. Is the weight of the boat accurately known, and does she trim level or stern down at rest ? It is always handy to have tabs anyway, so not a rash decision to fit them

 

Hello MIA,

This thread and your case has reminded me of very similar issue that I had and which was really decisive until I found out the root cause. Maybe it helps, maybe not, but I will share it, maybe it helps others, too. I am also of opinion that it is a weight issue.

My vintage boat is 9 meters long semi-displacment (8 meters on water level) and its weight is in the range of 4-4.5 tons in total. The engine is heavy old diesel with 200HP which weighs almost 1 ton with all accessories around it and is placed all the way at the stern to be connected to the stern drive. In perfect world it can run 37km/h max (even without tabs), but after a couple of months in the water and some growth or rather just a slime on the hull the speed decreases to around 30-31km/h and tabs become very useful for transition to planning. A few more months in the water without cleaning, reduces this speed to 27km/h and no help at that point without cleaning. After this, there is no much change. Since I drive it in displacement speed, I do not care too much about the reduced max speed. There is no change at displacement speed, maybe fuel consumption goes up for 0.5 l/h in the worst case, but I have not detected it. Displacement speed is around 11km/h and consumption around 4 l/h. Anyway once or twice per year I do this load test to clean the engine and to check the systems and to know the state of bottom. If you would push the throttle all the way up with the growth, engine would easily overrev, so I do not go over the max rated RPM and I already know the issue. Anyway, this is when all is normal and this does not refer to your current case.

Once during this load test it has become strange when the max speed fell down all the way to only 20km/h no matter what I try. Next year, I started with clean hull at around 30-31km/h and very soon it was again reduced to 20km/h. It did not have any sense. All the systems were working flawlessly, just the speed and planning was not there. And here is what was the root cause. During one pull-out of the boat on trailer, the boat speed sensor (you know the one with small gear rotated by the water, probably nobody using anymore since the GPS times, but I installed it for some reason, do not ask me why) lied down on the trailer carrier, and the sensor is not flat with boat bottom (vintage type sensor). What happened, and what I have detected at that time is that the shaft of the small wheel was broken/pushed in, but there are no leaks or anything else, so I simply removed the display instrument and stopped using it. A one or two years later though I have decided to remove the broken sender and put the plug instead. What turned out is that there was internal crack in outer "pipe" in which you place the sender or the plug. And the crack was such that it was leaking inside the fiberglass, but not inside the boat (so not easily spottable). And that was making a weight problem to the boat. Part of sandwitch core was wet, luckily nothing that was expensive and core was not affected by water (not softened, not disappered, interesting quality material). I had to leak/dry out that space and close the hole with fiberglass, no more sensor on that place and no more leakage inside and weight reduced.

This is a long thead, so I will not read all again, but I think you stated your boat is not sandwich structure, but in case I am wrong or you are wrong it is worth checking such stuff (water in fiberglass), especially since it is a vintage boat like mine. So my humble "opinon" is that it is weight and that tabs will probably help, but you will probably reach modest performance.

Other two options to consider are: 1. as mentioned by others, go with high power if possible and overcome the issues by brute force if finances allow it or 2. a) reduce the throttle, relax and enjoy the displacement ride (but I know this is the hardest for us boaters, we always have to be solving something if there is something to solve), b) change the engine for less powerfull one which is fit for displacement speeds and you have removed the issue that needs to be solved, so then you have no choice but to relax and enjoy the displacement speeds .

To be honest, I think the main reason displacement boaters enjoy their boats for decades without issues is because they are not being affected by a myriad of issues, so issues are not even spotted and even when they are, they are reluctant to fix it since it does make a little change on their usage. At least with bigger complicated boats, not mentioning dinghies here. Who knows how many people drive the boats with water inside the core for ages and then just sell it in the end to become the problem of someone else.

 

There was, in an early post, a picture showing the shaft and strut. It seemed to have a quite high inclination, do you have an actual number for it?

 

Indeed:


As the boat trims as it increases speed and the stern squats, this angle is exacerbated...

 

And the angle is already far far too steep.

 

That pic does seem to portray an extreme angle, MIA may be able to clarify the actual angle, though with the boat in the water may not be.

 

The other aspect would be the shaft angle in relation to the COG too, I guess, it is starting to sound like this shaft angle could be the snag. That would be a bummer.

 

First, thanks for the comments and suggestions, especially to Nidza who took the time to make some really thoughtful comments.

As for the shaft angle. That's a new one! I'll have to wait until fall to measure it. I can tell you that the camera in my Motorola phone may be distorting that angle. Those phones were not noted for their camera quality. The v-drive angle is 20 degrees and the engine sits level in the boat.

I'll post a photo below that I took last night. She's sitting right on her lines. That dinghy rack is a work in progress. See the leg on that outboard kicker? I notice if I stand right at the back of the cockpit, in the center of the boat, the bottom of the outboard leg will be an inch or two under the water. I weigh about 170 lbs. It seems to me that the boat is pretty sensitive to weight back there.

As far as the center of gravity.....Back years ago when I did the rebuild I moved the batteries forward. I use AGMS and they are in a locker under the cabin sole about even with the little air conditioner that you can see in the photo. I carry 30 gallons of water and moved those tanks forward as well. They sit slightly forward of the center of the boat. Both the batteries and tanks are right on the center line. I'd estimate the weight at 400 lbs. with full tanks. The original build had the batteries in the engine compartment and the tank under the cabin sole but right under the rear bulkhead. I figured she'd be better off with the weight moved as much to the center as possible and as low in the boat as possible.

As for Nidza's thoughts.....Yes, she's solid fiberglass. As for weight, if you look back to post #28 you'll see a link to spec for a Silverton 26 that's almost identical to mine. Between that boat and others like the Bertram's that are similar I derived an "estimated" weight of about 7,000 lbs. This should be close. I hope so anyway.

This is a hobby boat. Sounds like yours is too. In that respect I enjoy working on her just as much (sometimes more) than I enjoy taking her out on the water.
Re-powering her is an absolute last resort and probably will never happen, unless I have some kind of serious issue with the 302 Ford.
Replacing the v-drive, even though I've not solved the planing issue was certainly worthwhile. While the engine and transmission were in excellent condition the old v-drive has had cooling (raw) water pumping through it for 47 years. I notice the new drive is much quieter. When I'm far from home, whether she planes or not, I'm happy that I installed the new unit.
After I spent 7 years in the boat yard rebuilding and restoring this old girl, I've spent the past 7 years running her at displacement speeds. I'm happy doing that. She gets 3 statute miles per gallon of gas at displacement speeds and may get a little more with the new prop and drive. I notice engine RPM is slightly lower at displacement speed now.
The reason I decided to re-prop her was that taking her into the ocean, there are times when I need to transit inlets or perhaps outrun some weather. I thought it would be a good idea to have the capability to run her at, say 20 knots when it was necessary for short periods. So in that respect, even though the goal was to get her on plane, I didn't plan on running her fast for often or for long. So I'm with you on the slow and steady aspect. Less stress and much easier on the mechanicals.

Thanks to everyone who posted,

MIA



Regards,

MIA