Get out the Pop Corn!

[MOP]

I was talking with a fellow about having to vent my drive to get the boat to plane and how I was going about it. We got talking about through hull exhaust boats and wondered if removing the bellows to allow the prop to suck air would relieve the vacuum behind the hub. We all know there has been a few things done over the years to increase the suction to help pull the exhaust through, flairs and a foil shaped rings all to obvious benefit and proof of some pretty good suction behind the prop. So will removing the bellows there by relieving the suction really make any difference in overall performance? Ponder it a bit, don't be to quick answering I want to run and get a beer!

Phil

[Greg]

You may be on to something here Phil. The boat I am currently running has an '86 vintage 5.7 w/Alpha 1 drive. When it came from the factory it only had thru-hull exhaust. There never was an exhaust bellows installed. Since we changed out the engine and added silent choice doing a comparison is impossible. I'm waiting for the popcorn.

[marcdups]

Phil, according to the mercruiser manual for their quick and quiet exhaust system, they advise you to remove the exhaust bellows

Quote : IMPORTANT: When installing silent choice exhaust, it is recommended that the exhaust bellows on the transom assembly be removed. This is necessary to avoid creating a vacuum at the exhaust outlet in the propeller at higher boat speeds. This vacuum could degrade propeller performance on some boats.

[Greg Maier]

Phil,

I'm not sure why you feel there is a need to vent your props on the proposed setup of "The Blue Beast". My Blackhawk is not vented, is turning much taller props than you are expecting to turn, and I don't have a problem getting on plane.

[Greg Maier]

How do my props self ventilate? Please explain...

[mattyboy]

I was talking with a fellow about having to vent my drive .............................. I want to run and get a beer!

Phil

this fellow's not your bartender is he???? ;) :rlol: :) :)

Greg I think the point is your setup is a "true" BH setup raised X dimension surface drive with surface piercing props so the ventilation is an intergral part of that equation

MOP's setup is neither on all accounts

[Greg Maier]

Did Woodsy have to ventilate his props to get on-plane? I don't know the answer. But, he was using the stock x-dimension, same as what Phil will be using, I assume.

[boxy]

Did Woodsy have to ventilate his props to get on-plane? I don't know the answer. But, he was using the stock x-dimension, same as what Phil will be using, I assume.
but Woodsy was using BH props, not B3 cruiser props...

[Greg Maier]

So, let me get this straight.. You don't need to ventilate a 31 pitch surfacing prop, but you do need to ventilate a 24 pitch cruiser prop (assuming the same gear ratio of the drive)... Seems kind of backwards to me

[boxy]

I agree....... :D

[MOP]

Did Woodsy have to ventilate his props to get on-plane? I don't know the answer. But, he was using the stock x-dimension, same as what Phil will be using, I assume.

Greg from what I can see I will be 3" deeper in the water than the BH boats similar to the first few BH boats, it will be harder to plane then a long leg. I forgot who but one of the BH owners said it was slow to plane with the exhaust going out the transom but came up fine with the diverters closed and going through the drive. I need reasonably good planing as I could have 4-5 people aboard at times when out showing boats. I have done over a years worth of research on surface piercing, getting the right input from builders and suppliers of several different type drive systems. And quite a bit from those that have gone to various surface piercing applications. It makes a lot more sense putting the vent in now rather than trying to do it later after the engine is in. I will still have all of the blade above the gear case out of the water when on plane, difference a properly setup BH has just the fin an blades in the water, the gear case rides just clear. My gear case will be in the water but the blades above the gear case will be surfaced, I will be running in a much more surfaced mode than most shorty's.

For those that have real interest below is some very good reading to get a good understanding of surface piercing.

http://www.simplicity-marine.com/surfprop.htm

[mattyboy]

great article,
explains the different pressure asserted on a surface prop could that explain why the BH props are prone to cracks??
also explains that prop design for surfacing is a big factor, and if it is above water for half of the revolution it is surfacing, and how surface props ventilate and none surface props cavitate so Phil if the blade will be above the water on plane what will the B3 props do??
I would have atleast tried the B3 props and the BH drive as a test run before doing any glass just incase adjustments need to be made
also maybe you can incorparate a vent into the back seat when you re glass it and plumb it to the drive????

[boxy]

also maybe you can incorparate a vent into the back seat when you re glass it and plumb it to the drive????
Maybe you could glass in some sort of seat that you could raise and lower. Raise it for take-off to ventilate the prop, then lower it once you are under way...

[gcarter]

The book by Sonny Levi I recently found discusses a similar problem. In the mid '60's he built a turbo diesel powered boat w/ a V-drive. The original prop didn't allow the turbo to spool up so they included a compressed air tank to vent the prop. Once the boat planed the air was turned off. But the real solution was to eventually find a prop that worked.
:wavey:

PS; My avatar got squished!!!!! I guess I'll have to resize it.

[Bad-Tat]

Guys,
The Blackhawk exhaust doesn't go thru the props when diverted down. It exits thru the grill above the gear case in front of the top half of the props. I have found it comes up on a plane much faster when the exhaust is down to vent the props. If you look at some of the offshore boats they have a large tube placed vertical on the cavitation plate to get vent the props so they can get up on plane.

[MOP]

Now that we may be getting past letting the drive suck a little wind! :jestera: Lets go to "props" early on in this foray I said I would probably have propping issues, more with configuration then pitch. My memory being bad :confused: one of the guys (think it was Ted) was kind enough to speak with the tech's at Mercury :wrench: about what I needed to spin the BH 27's they said about 370HP. I think I am close to that, anyway I missed getting the 27's BH's that Chris got with the drive. That is when I got hold of Dave and found out he was going to try BIII wheels when he ran this drive but they needed to be machined and his plans were changing as we all know!. I ended up talking with my :propeller guy another old timer like me he said the BIII's are a pretty heavy prop and should be OK for my app and the way I run. So I figured the BIII 28's would be a little less to twist then the BH 27's and I grabbed a set real cheap and had my machinist fit them to the drive :wrench: (pretty minor operation to the front wheel). My :propeller guy wants to see how it runs with what I have then figure out what needs to be done :smash: I told him about speaking with Geo about him needing to go to higher rake to get more bow lift. He said that will help with mine also but I that I was stuck with the front prop unless I wanted to part with $$$ but he could work on the one I have then showed me some wheels that with a little machining could be used on the back that should help with bow lift. So here I sit with a lap full of supposition which can be answered by getting it wet in another couple of months. Those that took the time to read the surface drive thing I posted, may have noted there is little difference between surfacing and fully submerged props. They run the gambit from cleaver to round ears 3 through 5 blades. Guys that have run OB's with jack plates know you keep going up until the engine can't suck enough water to keep the power head cool, I ran a few over the years with of the shelf wheels and spun them at some scary RPM's with never a problem they were running surfaced that for damn sure.

[mattyboy]

Those that took the time to read the surface drive thing I posted, may have noted there is little difference between surfacing and fully submerged props. They run the gambit from cleaver to round ears 3 through 5 blades. Guys that have run OB's with jack plates know you keep going up until the engine can't suck enough water to keep the power head cool, I ran a few over the years with of the shelf wheels and spun them at some scary RPM's with never a problem they were running surfaced that for damn sure.

Phil,
what I got from it was that surface props do come more in 3 or 4 blades and round eared than the cleaver and are hard to distingush visually from submerged props but that there are basic design features that are different ???



from the article

Propeller Selection: Surface propellers are usually associated
with the stainless steel "cleaver" style common to race boat
applications. These propellers have straight trailing edges,
razor-sharp leading edges, and sometimes as many as eight blades.
Probably because the roots of surface propulsion technology are
so firmly imbedded in the race boat world, it's no surprise that
the popular perception is that all surface propellers are
cleavers. Yet the vast majority of surface propellers being sold
today have round-tipped blades, are made of bronze (or NiBrAl),
and have only three or four blades. In fact, at first glance
there is very little to distinguish them from conventional, fully
submerged props.

What distinguishes a surface propeller from an underwater design?
The pressure face of the blade is always concave, the leading
edge is relatively sharp with a narrow entry angle, and the hub
and blade root are built to withstand heavy eccentric and
alternating loads. There is major incentive to keep the blade
section thin (it's the strength of the steel blades that really
gives cleavers the edge at high speeds and loading). Nearly all
successful designs have moderate to heavy trailing edge cupping.

does the Bravo 3 have those characteristics???
were those OB's running Bravo 3"s ???


I guess you're not going with a switchable exhaust and that's why venting is an issue??

[MOP]

*does the Bravo 3 have those characteristics???
No but are fairly heavy originally designed for cruiser, but as Poodle and a few others have pointed out are showing up on Hi Perf boats. What am I putting together?? Yup a cruiser for work and play. I will not spin the engine over 45-4600 WOT just a little above my peak torque curve, the props with the 1.5 gears will be spinning them just under 3,100 RPM not much sweat there. Ye! forget I am shooting for 45-50 at reasonable RPM the math shows at 3,350 picking a middy number the props will be spinning almost 2,500 RPM. Every post I have done on the swap has more than clearly stated there will be propping issues I have no problem with that it is part of seeing IF The Experiment can be made to work! If we all sat around worrying and not trying we would still be living in caves! Sorry but I have the moxy to try!

*were those OB's running Bravo 3"s ???
No some S/S and some aluminum are weaker but held up on the light hulls I ran!

*I guess you're not going with a switchable exhaust and that's why venting is an issue??
My headers end to close to the transom I can not run diverters nor do I want them, did get Gil slip on mufflers to tone it down a bit. By the way what do diverters have to do with venting the props???? :confused:

[mattyboy]

Phil,
if you not sure how the props will work how are you so sure they need to be vented???

If you read Bills post he affirmed what you had said that with a standard BH it comes on plane quicker with the exhaust switched down hence ventilating the props, so if you had diverters the need for the discussion would be mute.
and I see no need to add non pertinant facts as too how an OB on a lighter hull runs we are talking about a 22 classic with modifications to it's center of gravity , a Black hawk surface drive at a lower X dimension with non Black hawk props

I don't mean this to be confrontational but the forum is an exchange of ideas
and we are all learning from this and every point has a counterpoint, or old school and new school however you want to say it. if you are not looking for alternate points of view perhaps you should not name the post "go get popcorn???" regardless these posts will be looked at by newcomers or surfers in the future and they need to see the question why did you cut the seat to get the new tank in?? why not lift the deck??? why not move the X dimension?? why not Black hawk props why shift the weight, so that they put in a similar situation can have enough info form an opinion on what's best for their boat?? and don't forget the fact that when you posted your thoughts on this it was some 1000 posts ago we now probably have that many new members so they are not familar with your experiment or the reasons why or the outcome you are looking for.

so if you not looking for a two sided discussion, or an alternate point of view let me know I know when to shut up

[MOP]

Matty remember I am starting with a boat that needs gobs of trim to get it to run decent, even if I made no other changes power or drive I still would have gotten weight aft. Alone and on a 1/4 tank it ran 61.9 which several have said was pretty darn good, with a full tank it would hover right at 61. That tells me and others the boat was setup wrong to begin with, it rode way to flat. With the changes I am making my CG will still be forward that of a late model 22 and so even more weight may need to added only wet time will tell what is needed. It is obviuos Donzi saw fit to move the 22 CG way back definitely more then mine will be for now, if moving the CG aft was an issue then the late boats would not run well and they run damn well! I would really like to see the results of 22's running exact power and props, an older tank forward app "vs" a late aft tank. My opinion the tank aft models which also have their batteries aft will be a tad faster needing less trim to get the bow up. Having no rocker my only options are weight aft and prop experimentation. I will not spring for Rocker Plates, to expensive for my pockets!. Geo has all his weight aft plus the Rocker plates to get the lift he wanted, and now can induce porpoising. At 120 MPH I know he is doing it right he has helped with good advice. I am hoping to get at least some with the lower X, weight and some fiddling with the props, again may or may not work we all know that!

*if you not sure how the props will work how are you so sure they need to be vented???
Surface piecing props on no matter what hull or engine combo are still surfacing and may depending on app may need venting, I have studied many different surface applications most hi perf boats need venting.

*If you read Bills post he affirmed what you had said that with a standard BH it comes on plane quicker with the exhaust switched down hence ventilating the props, so if you had diverters the need for the discussion would be mute.
How so whether straigt or rockered hull the props still need airing out for reasonable planing, mine being deeper maybe even more so getting air to them is a must either by the exhaust or just allowing it to draw air through a tube.

*I see no need to add non pertinant facts as too how an OB on a lighter hull runs we are talking about a 22 classic.
Why not when talking about how a prop will run with either raised X, jacked or with a shorty. Most all surface to one degree or another with some of their blade breaking the surface. It has been said so many times people forget to use the search button that is where tons of info good or bad can be found, it is up to them to TTT or chose what is or is not good.

*I don't mean this to be confrontational but the forum is an exchange of ideas and we are all learning from this and every point has a counterpoint.
"Exactly" how can I or anyone else learn if they do not try! Point none of us will know anything about how this will come out until spring, from that we will learn if this can work. The final result is what causes us learn not the thoughts and experimentation!

*why did you cut the seat to get the new tank in??
Had to the box extended almost down to the hull it would be impossible to get a tank in. The plan is to shorten the box depth wise, it will end up just a few inches deep but give me some small stuff storage. I am adding transverses off the new stringers with platforms for what needs to go back there and some added storage.

*why not move the X dimension?? why not Black hawk props why shift the weight?
All but the BH wheels I think is covered, If possible I will run cheap BIII's. I would like to try BH 27's one day to see the difference if this thing turns out OK would I buy a set more than likely not! Like I said before if it does not do what I want or just plain don't work out I will strap on a BIII and sell the BH.

Damn watched two movies through this post!!!! If I missed something let me know LOL!

[mattyboy]

never mind :rolleyes: I'm missing something