Propeller truth,facts and fiction...

[2FORCEFULL]

Seems that there are lots of opinions on how propellers work, what works ,... and what doesn't, that being because of the so many different variables ..."X" dimensions one of the big ones... bottoms surface , hooks and rockers...what the boat was designed for...some boat just plain ol have a max speed...

in this thread I will try to seek out those that really know what they are talking about, and hopefully those that what answers to what they wanna know also..

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Defuser ring facts,,...

defuser rings were designed for through hull exhaust boats.... it is fact that they cause a loss of top end speed, loss of 1 to claims of 4mph can be gained by running the same prop without it...

but...some boats struggle to get on plane... so the ring helps the prop bite better on take off

the ring design stops air from getting forward to the blades...causing a cavitation ....works kinda like a stall converter on a car... without the ring it's like trying to go up hill from a dead stop in the rain... or worse yet.. on ice... so...like a car with different tires....props play the same roll..

getting a boat propped right is a very fruitful experience...gratification is there with a great big smile ...

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prop size and pitch. major difference in performance... getting it right takes some time and knowledge.. I fine tune with a vacume gauge ..

I've heard people say,... I run this prop because I can cruise 55 at 3000 rpms... but it will only go to 4200 on top end..... that is so wrong...at 3000 rpms you are sucking more fuel then you would at WOT.. it's like a motorhome going up a hill...you can get real close by just listening to the motor.. is it singing to you, or lay'n down growl'n. with a vacume gauge you can juat look to see if it making vacume.. my 270 Hallett with twins has dual vacume gauges...they say, GPH , but are just gauges that monitor motor vacume...at WOT they say 40 GPH... let off and they go to 20 GPH...I could be wrong,.. but pretty sure they don't actually measure fuel used..

out drive boats are limited to just one gear, you use the same gear to take off, as you use for top end.. they don't have a clutch to slip like a old front engine dragster.... so the prop has to slip... the idea is for the prop to slip just enough to get the boat on plane, but not enough to cavitate and blow out..again, just like a dragster..you blow the tires off it and you go no where...same again as a prop, my dragster would start hazing the tires on the big end ...same as prop slip on a boat.. so again refferance is the same... you need a good clutch guy for a dragster, and a good prop guy for your boat...

three things to look for in a prop...take off , cruise, and top end..you want to prop for max rpms, beable to cruise, but most of all... be able to get on plane..

boat builder seem to send a lot of boats out with what ever prop they have on the shelf that is close... or even wose... what ever they have in stock... I heard owners say... this is the prop they told me to use...

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so now with the great debate... twin motors, and prop spin direction... some say that deep V's should spin the props in... and some say spin out...and they say the same for cats...
so I guess the only way to know is trial and error?

My Hallett 270 spins them out...when and what would be the reason for the way they spin, what do you look for in performance to decide if it's right or wrong??

my Hallett w/twin 400 hp has bravo 1 24 p 4 blades. w/ defuser rings.. gets right on plane... I added 1.5 drive spacers.. it's a rocket out of the hole , and really pulls hard in the mid range...it is real sensitive to trim.. a little over neutral is all she likes...like 1 number over... I don't really have enough seat time to make an assessment as to how it handles at top end, because I have spent so much time driving the wild cat deck boat that is so stable at all speeds... the side to side rocking of a deep v is something to get used to I guess, but can be very alarming.. the hallett starts to dance a little after the mid range... could be the defuser rings are making the props try to climb to the surface...I don't know is my answer... but will find out..

[HB Vic]

I'll get to props in a min, but the whole where do you run your drive perplexes me. I mean every boat is different, unless we have the same exact setup with same trim indicator, its kind of irrelevant isn't it? The boats would have to be a mirror image of each other for this question to mean anything.

But I could be wrong

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understanding prop pitch....the fastest you could go with a 24 pitch prop, a 1.50 gear @ 5000 rpms is 76 mph...so when a guy says his boat does 80, my eyebows raise...
that would be at zero slip!!!and would be at 5280 rpms... most merc motors max right around there.. now throw in 15% slip @ 5280 rpms... the speed drops to 68 mph..so to go 80 mph the motor would be spinning 6212 rpms.... not in a stock merc power!...aint gonna happen

I have never GPS'd my hallett... and have no idea what the slip is... it has a 2 step bottom, so it has to be better than a straight bottom? right or wrong???

it feels like it's hauling ass... but I don't have facts.. so won't say how fast it goes

there are 5 things to know about prop speed..you have to know 4 out of the 5 to calculate,.. the 5..gear, pitch, rpm,slip= actual speed..for most... who cares right..
but for gear heads... the I gotta know!

you can do it with three to get a guess number, just plug in a slip number... then go out with a gps and find out the real slip number,..

it has been said, to find out how fast the boat goes, keep trimming up till you loose speed,... or crash.....so be careful if you try this method

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I'll get to props in a min, but the whole where do you run your drive perplexes me. I mean every boat is different, unless we have the same exact setup with same trim indicator, its kind of irrelevant isn't it? The boats would have to be a mirror image of each other for this question to mean anything.

But I could be wrong

as in "X" dimension? or trim angle?? I always straight edge the drive to the boat to see where neutral trim is, on my hallett I marked the gauge... all boats should perform the best at neutral trim... and should gain top end speed trimmed up...with a lot of boats, the "X" plays a roll... neutral trim at cruise speed cause's the boat to porpoise

most boats have reference gauges... not accurate gauges..

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it has been said many of times.. "a guy that thinks he know something is dangerous".... I'm a dangerous MO-FO... but eager to learn...

[HB Vic]

as in "X" dimension? or trim angle?? I always straight edge the drive to the boat to see where neutral trim is, on my hallett I marked the gauge... all boats should perform the best at neutral trim... and should gain top end speed trimmed up...with a lot of boats, the "X" plays a roll... neutral trim at cruise speed cause's the boat to porpoise

most boats have reference gauges... not accurate gauges..

I was talking trim angle, but the same can be said for X dimension imo

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so,..... here's where i'm at.... I added the 1.5 spacers.... the optimum drive setting when building a boat is to set the drive to where the lower cav plate is at water line (see pic) what happens though is as you go more positive trim, the plate creates drag...