Beck boat rebuild needing help

Could your problem be as simple as weight too far forward? A lot of low powered planing boats can not get over the hump because of that.

It looks like you have a small diesel mounted quite far forward. If you helm is not way far back you are going to have a heck of a time getting that boat up on the plane even with a lot more horsepower. I remember that a lot of 25-30hp outboards of the early 1950's had to have the helm in the rear cockpit to get up on a plane. Remember, those had the engine mounted behind the transom.

 

Most everything is really centered in the hull. The original Briggs engine was actually mounted about a foot further foreward than the diesel I have. Ill try putting more weight In the back Sunday or Tuesday when u get more time to mess with it again.

Today I managed to borrow a 4 blade 13.25x16 prop and ran it today however she only made it to about 2000 rpm again. I have noticed something very strange tho and perhaps someone has an idea why it's happening. Originally I ran a 14x10 3 blade dyna jet with no cup and the boat had a horrific vibration when in gear all thru the rpm range up to the redline. Like a teeth rattling vibration...
Then I swung a 16x16 dyna jet with no cup and the vibration was gone completely so I assume it was a bad prop (the 14x10). I swung a 15x16 cupped and again no vibration so in my mind that confirmed the 14x10 was messed up. Until I had the 15x16 re pitched to 15x13 with cup I noticed the vibration is back. Not as bad as the 14x10 but certainly something very noticeable. Today I swung the 13.25x16 4 blade wheel and I get no vibration... Anyone have a theory or reason for why this is happening?

 

The vibration could indicate that the prop is too big for the aperture. Usually need 15 to 20% of the prop diameter as clearance to the closest part of the hull.

That might be 2.5 to 3" of clearance for your 15" prop.

It may be that the bigger prop did not get fast enough to reach the harmonic until you reduced the pitch.

:?: Just my 6.35 cents worth (2 cents plus taxes)...

 

im pretty positive i have the clearance from the prop tips to the bottom of the hull. even swinging the 16" wheel with 16 pitch ran perfectly smooth all the way from idle to 2000 rpm. and that's the largest wheel ive gotten my hands on. (im pretty sure 16" is the maximum itle handle)

As for the harmonic thing i honestly don't know what causes that or how to fix it but i do know that the 14x10 shook as well as the 15x13 from idle all the way to their max rpm with the vibrations getting worse as rpm increased. where the 13x16, 15x16, and 16x16 never shook anywhere in the rpm range. (and the 15x13 is the same prop as the 15x16 just repitched)

 

There is a whole can of worms there, the 15 - 20% guide is just that.

If you look at a boat with the prop in a nozzle, there could be an inch of clearance on a 50" prop - so only 2 percent...

The issue is that cavitation, pressure waves and vortices shed from the tip of one blade are reflected back from the hull towards the next advancing blade - they could miss it completely depending on the pitch and the shape of the hull, BUT there is also a possibility that they could reinforce the ones being generated by the oncoming blade - these forces grow rapidly and without the 15% clearance for them to disipate they cause vibration of the hull.

And of course the nearer you get to the sweet-spot the greater the chance - I take it that the 16x16 didn't give you what you're looking for... It wasn't putting the power in to the water so it didn't generate the side effect.

Over time this can actually erode the hull.

While it is 'science', there are so many variables that unless you wan't to fork out for some serious computer modelling, it might just as well be voodoo. Don't get frustrated just keep trying and be as subjective as possible so that you know if its getting better or worse.

 

yeah the 16x16 was overloading the engine but it was smooth all the way up to 2000 rpm which is where the engine maxed out. where the lower pitch props were creating heavy vibrations from idle to the max rpm that they would allow the engine to produce.

do you know if it would be possible to alter the pressure waves to change their timing by say having the tip geometry changed on an existing propeller? for example the closest ive gotten to the right propeller is the one that is 15x13 cupped. if I were to reduce its diameter to say 14" to make it a 14x13 do you think that would help solve the problem? or is it not really possible to tell. just one of those try it and see if it works type things...

 

Maybe worth posting a pic - the collective hive brain here on BDN may spot an issue.

If you have some 'end of life' props, yes cutting them down as an experiment may tell you what you need to know - and those guys are often available for about what the scrap man will give you when you weigh them in after playing.

You can put a scrape of filler on a badly pitted 'dog' and hand sand and it will stay on long enough for a sea trial.

I'd measure the aperture or tip clearance and try and get an idea of the best diameter then work the pitches on a constant diameter. You're changing both pitch and diameter which makes it harder to draw meaningful data... but e.g. the 16x16 vs. 15x16 vs. 13.25x16 should have been informative?
16 pitch is clearly too much for the motor, but 10 and 13 vibrate, Hmmm...

It may be cheaper to have the 14x10 repitched and balanced to e.g. a 14x14 than to cut down the 15 if you're going the machine shop route, but the small adds and craigslist might also yield cheaper options.


Or of course you could get some really fresh goat blood and a couple of black candles

 

I have exactly 3-1/2" of tip clearance with the 13.25x16 inch wheel that's on there right now (that's the 4 blade one) the other picture is the 15x16.

16" pitch props seem to be more abundant around here for either really cheap or people letting me borrow them so that's why I keep trying them...

I have about 20-1/4" from center of shaft to the bottom of the pocket.

ill ask the propeller people if they can pitch the 14x10 up to 14x14 but last time I spoke with them they will only go up 2" in pitch and down 3"

sadly there aren't many goats In the area but there are pigs and coyotes. do you think one of those along with a dark red scented candle borrowed from my wife would suffice?

 

Looks like you're on the right track - the 13.25 is about 26% clearance so you can go a little bigger...

The issue is the pocket is wedge shaped which makes things a touch harder, but it may be very worthwhile making a cardboard template for the guys at the local prop shop, if you ask them to mod any of the props (without the tapering pocket, a 16" prop in a 20" hole would easily work) they may be able to trim the leading edges more than the tips once they know the shape of the pocket.

The 4 blade looks like a fast running prop - e.g. for a ski boat, the 3 blade looks like the blade style for a boat like yours.

I also can't tell how much clearance there is to the rudder?