Choosing best prop/rpm

[G DANE]

After a long year with tearing down walls and installing new kitchen a.s.o. I´m back to start working on my project boat again. I have a 1991 115 Evin and a 17' runabout that will weigh 1500 lbs with motor, gear and crew. Deep V. Using JB´s patented speed formula, I should be able to do around 50 MHP. Motor came with a brand new 15" Solas sst prop that I cant use. As I have to watch my $, I count on finding a new prop on ebay, and will try to get the best from start. The question: 19" pitch and 15% slip ( which I hope to be limited to ) will give me 46 MHP @ 6000. 21" pitch should give 51 MHP @ 6000. Is it better to prop low, and turn above 6000 now and then - 19 shoud lead to 6600 RMP ?? Will it go that high without any harm. Do I have to remove the intake divider to get that high ? Boat will be used for as well cruising as some skiing.

 

[Cricket Too]

Re: Choosing best prop/rpm

G DANE.....Don't have an answer to your question, but what exactly is that formula of JB's that you spoke about, I'm interested in using for my setup. Thanks.

 

[walleyehed]

Re: Choosing best prop/rpm

This should be close, and 12-14% would be a realistic figure for slip, I think.<br /> http://www.rbbi.com/folders/prop/propcalc.htm <br />That being said, I think the 19" will be your best option in my opinion.....

 

[walleyehed]

Re: Choosing best prop/rpm

@15% slip, here's what I figure:<br />17"-39.1mph@ 6k<br />19"-43.7mph@ 6k<br />21"-48.3mph@ 6k<br />@12% slip;<br />17"-40.5mph@ 6k<br />19"-45.2mph@ 6k<br />21"-50.0mph@ 6k<br /> I think you will find the 21" will not turn 6k without some tweaking, depending on the prop you choose and engine heigth, and possible set-back....but hey, I've been wrong before!!!<br />Maybe Dhadley will provide some insight here.<br />Good luck!

 

[G DANE]

Re: Choosing best prop/rpm

The formula was:<br /><br />Prop HP divided by boat weight. Take the Squarroot of the result and multiply with:<br /><br />Flat bottum 220<br />Nearly flat bottum 200<br />V bottum 180<br /> <br />Result is MHP at WOT - it really works - I tried it on my old boat - was within a mile.

 

[G DANE]

Re: Choosing best prop/rpm

What do you think of running above 6000 rpm ?

 

[Dhadley]

Re: Choosing best prop/rpm

The stock reeds will start to flutter in the low to mid 6K range. It depends on the reed stop height and shimming. The stock engine components will work fine thru 7K except the pistons. Personaly I hate to turn a cast piston over 6500.<br /><br />As for the prop, I prefer to aim to the lower pitch and then add "bite" if needed. Its a lot easier than going with way too much pitch and having to decrease "bite". Increasing slip % very seldom makes the boat faster.<br /><br />Good luck!

 

[G DANE]

Re: Choosing best prop/rpm

Thanks all for your help - I will start with a 19" then. I just saw an old OMC prop chart, recommending 13 by 19 up to 50MPH on light boats.<br /><br />I calculated a 19" at 15 % slip/6000 Rpm to 46,14 Mph.

 

[LubeDude]

Re: Choosing best prop/rpm

Got in here a little late, but you got the advice you needed!<br /><br />But knowone mentioned the hight and position of your engine on the transom, (but then you didnt ask)! Your speed will be greatly effected buy that! You can experiment with that when you get on the water! Walleyehed has done great indepth tests with this! Hes the man!!<br /><br />LubeDude

 

[G DANE]

Re: Choosing best prop/rpm

Thanks Lubedude<br /><br />I am planning to start with antiventilation plate 1" above keel line. As this is a deep V I am afraid it will ventilate quite easy in turns.

 

[AUGIDAWG]

Re: Choosing best prop/rpm

Get a 4 blade to prevent the ventilation.<br />Get the 19" pitch.<br />They do make some 20" pitch Turbos.

 

[WoodsJ]

Re: Choosing best prop/rpm

GDANE,<br />Have you weighed the boat and motor? I would think a 1500lb total weight is too light, especially since you have a deep-v boat. The motor and one passenger would be at least 600lb of the total weight. If this is the case your formula would be off.

 

[G DANE]

Re: Choosing best prop/rpm

WoodsJ<br /><br />I only weighed the empty hull. I took over this as a new hull that was never finished. I put stringers in with supervision from a local boatbuilder. I too added one layer of mat to the buttom. The empty hull weighs around 450 lbs. Average weight will be: hull 450, windshield and seats 150, gas and tank 80, motor 300, crew 500.

 

[walleyehed]

Re: Choosing best prop/rpm

G DANE, I've built 3 deep-v's now, and I can tell ya first-hand, you better add about 350-400lbs. to your figure. My 17ft starcraft came out weighing 522lbs more than I figured....my first 18ft'r came out almost 400lbs. higher, and the last 18ft I rebuilt, I got within a 100lbs. of my figure, but it was still heavier than I thought it would be.<br />Good luck and let us know the weights! <br />p.s. The 1" figure for cav plate is a good starting point, and I think you will not see much ventilation at that heigth, unless trimmed extremely high, which would indicate a need to raise the engine more, requiring less trim.

 

[G DANE]

Re: Choosing best prop/rpm

Thanks Kenny.<br /><br />When I weighed the empty hull ( upper and lover section, floor is part of upper section, all glassfibre ) I was amazed it was only 350 lbs. I think I added aro 60 lbs of poly and 50 lbs of wood for the buttom structure. Isnt that a very light hull ?

 

[walleyehed]

Re: Choosing best prop/rpm

G DANE, that's VERY light!<br /> I've got my 115 mounted about 3 inches above the bottom-line, but I'm 6 inches back...can't tell ya how big a difference that made!<br />I'm running a Raker in 18" and getting about 5700 with it. I've run 27 different props on this boat and I've tried the 4-blades, as AUGIDAWG likes, in Turbo, Stilletto, and PowerTech, and they just don't have the speed in my case...Matter-o-fact, I tried 3 different Powertechs, and the Flats series is the best in that group, a very good prop, but 2-3mph slower than my Raker 3-blade. With tuned exhaust chamber, Boyesen reeds, and some port work, I CAN"T drop the hammer hard, it's got more umpfff than my transom can stand, I think.....hole-shot and mid-range are the least of my worries.<br />One prop I DID NOT like, is the Ballistic...It will blow-out much sooner than most of the SS props I've got and the blades are soooooooo thin, and yet, not as effecient as I had hoped.<br />I couldn't find anyone who wanted to rework a Ballistic either???? Pitch blocks uncommon, I guess. The balance point of your boat will determine the rake needed when you get the finished product done.<br />Keep us posted, and If I can help, give a holler!

 

[G DANE]

Re: Choosing best prop/rpm

Thanks all for your replies.<br /><br />Now the weather is getting real cold here. I will spend winther hunting prop and equipment, and be back with news in a couple of months.

 

[G DANE]

Re: Choosing best prop/rpm

Great to share your experience !!! Thanks.<br /><br />I'm at sealevel and my motor is a 1991, Proprated. The boat is 17' , but only 6,5' wide.<br />Do you think that would make the 21" a better choise ??

 

[walleyehed]

Re: Choosing best prop/rpm

G.DANE, If it were me, here's what I would do......Put a 19" on it, test for rpm as a starting point. If the R's are Too much (over 6000), I'd try the 21", but I personally think you would be better to add what is needed to the 19" to get them down alittle....

 

[G DANE]

Re: Choosing best prop/rpm

Guess I'l do that then - I just dremt of exceeding 50 MPH <br /><br />Thanks for shearing your experience - I'l be back