Would a 4 blade prop help me go faster?

[lincoln72001]

I have a 19' Sunbird 8' beam and a worked 2.4 Merc on it and a jack plate. I am currently running a 3 blade 14.50 x 19 SS and the boat runs 62 on GPS at 5900 rpm and it cavitates until the boat sets just right at full throttle, then it grabs and goes. It seems to me that the jack plate is in the perfect spot for top speed, I have messed with it and it's in the best spot, can I go with a 4 blade to help loose the cavitation and keep a bite. Does anyone have any advice on the 4 blade and am I running a good pitch? I appreciate any info and I will have pics of boat posted in a few days.

It's a Ballistic Stainless Steel 3 blade prop- 14.50 x 19
Boat runs 62mph on GPS at 5900 RPM, motor should safely turn 7500 rpm. 1.78 gears in lower unit.
Let me add it's bored .30 over, heads shaved .10, vertical reeds, aluminum flywheel, exhaust ported, intake ported, and it's on a jack plate.
This motor came off my hydrostream, not really set up for the sunbird but it's a nice boat with a fast motor on it, just looking for a little more speed.
I'm running in salt water, there is a current typically 3 to 4 mph, the boat runs top speed against the wind so i'm assuming that the wind is picking my bow up a little more that's another reason I was asking about the 4 blade, naturally I keep the motor trimmed all the way up.

[1BadAction]

I am currently running a 3 blade 14.50 x 19 SS and the boat runs 62 on GPS at 5900 rpm

numbers dont add up.

[150aintenuff]

they do at 1% slip... perhaps there are a few variables being left out... like down stream with tail wind and tach may be off... and prop could be more pitch than stamped... it is possible.... but not probable....

[Ziemer]

they do at 1% slip...

That doesn't add up either... 0% slip is 56.76 with a 1.87 ratio

[pyro]

What prop are you currently running? make/model

[Ziemer]

Maybe he really meant 52 mph and not 62 mph. ;) 4-Blades are usually faster in the midrange and are usually more efficient, i.e. more mph at same rpm...but that doesn't always mean they'll be faster on the top end because you will usually lose some rpm compared to a similarly pitched 3-blade.

[whipper]

You must be accurate with your rpm and numbers for anyone to help you.You really cant go buy your speedo! Get or borrow a hand held GPS and post the true speed,RPM and what your max rpm for your engine is. A 4 blade with this type of application can help you in a lot of ways. You really shouldn't loose to much top end maybe a mph or 2. In order to know what pitch would be best and even style of 4 blade the true numbers must be posted. There is a calculation for this and the guys are giving you a hard time because the numbers do not calculate in the slip factor realistically. Your hull and slip with the efficiency of that design will be above 10 for sure. So in order to make calculations GPS and RPM. ;) James

[150aintenuff]

That doesn't add up either... 0% slip is 56.76 with a 1.87 ratio


oops I typo's the calculator.. clicked on 20P=62.15 mph.. 19P cupped prop =59.16 MPH... figuring 52mph would equal 13%.... not to bad for a heavier boat....


as for his GPS numbers they are possible if he has a 10 KT current he is running with..... I know I can get 60 MPH over ground on GPS up at bonneville dam in my fissin boat when it only runs in the watter at 48 mph.... the current pshing down streem adds in the rest... so it isnt so far off that exterior factors that could plague the readings arent possible, if it were a lake with zero water movement i could see it being un realistic but we also dont know the envionment he was testing in..... or his current model and design of prop or if its been worked for more pitch either...


also dont forget to add in an inch on most calculators for cupping and rake changes on progressive pitched props..... if he runs a mirage its a 3"-4" progressive pitch so it could infact be running like a 20 or 21P....he also could have an XR4 geercase with 1.78 geers.....

[1BadAction]

oops I typo's the calculator..

abba ddeeddee duuhhhdd deee... you have more excuses than any human should need.

NUMBERS. DO. NOT. ADD. UP.


his tach is probably on the wrong pole setting or hes full of sh1t. call me a pessimist.

You must be accurate with your rpm and numbers for anyone to help you.
yep, we could help then instead of guessing at it like 150aintenuff.

[150aintenuff]

abba ddeeddee duuhhhdd deee... you have more excuses than any human should need.

NUMBERS. DO. NOT. ADD. UP.


his tach is probably on the wrong pole setting or hes full of sh1t. call me a pessimist.

yep, we could help then instead of guessing at it like 150aintenuff.

jim I didnt guess... i just added in a few variables that we havent heard yet that COULD make it possible to see 62 on a GPS with his particular proppeler......

oh wait your to dense to see that I am defending this persons information while also saying the data is a bit innacurate but if he indeed did get those numbers something he told us is incorrect somewhere... wether he realizesit or not.... it isnt our place to say this guy is stupid without first gaining more info.... he didnt say his gearring so we can only assume its 1 of 4 ratios.. most commonly 1.87's... so stop with the insults already... it gets old...

[lincoln72001]

I added more info for everyone. Thanks for any help.

[150aintenuff]

I added more info for everyone. Thanks for any help.


GPS number.... was it taken going down stream...... because with subtracting the current 4 mph I get 7% slip.....

and since a stiletto is a 3" progressive from 17.5-20.5P + cupping it is totally possible for it to run those numbers. sounds like its set up prety well... only thought would be to swap to a hydraulic plate so the prop can bite more on holeshot, they arent the greatest for semi surfacing operation.... but apparently yours is hooking up fairl well on top......

[INXS]

I've got one, the TR3 I think it's called, anyway I've read that they are within 1 mph of the top speed of the three blades?:confused: I was just wondering cause if you google it there's a comparision of props and i was wondering if it's BS or are they that good? I know they handel very well on my 21' Starflite, no chinning!

[150aintenuff]

TR4's are good as well..... add that to the list of props i PM'd you....BTW WELCOME TO SCREAMANDFLY!!!!!!!!!!!!!!

[1BadAction]

Boat runs 62mph on GPS at 5900 RPM, motor should safely turn 7500 rpm. 1.78 gears in lower unit.
Let me add it's bored .30 over, heads shaved .10, vertical reeds, aluminum flywheel, exhaust ported, intake ported, and it's on a jack plate.

ok, heres the first issue... with the small gearcase you have 2 problems. #1 on anything but a superlight boat, they are pigs, its hard to get a motor to spin hard with that tall gear (its like 2.43s in the rear of a car) what aggravates this, is the motor is opened up, which hurts the bottom end torque on a heavy hull. #2 the small case has less bow lift than a standard 200 case. do you have a cone on it? you can always fix these problems by running a smaller pitch prop and having it worked to give you a bit more bow lift.



the boat runs top speed against the wind so i'm assuming that the wind is picking my bow up a little more that's another reason I was asking about the 4 blade, naturally I keep the motor trimmed all the way up.

how much setback do you have? what do you mean by "trimmed all the way up"? where are the heavy weights in your boat? Ideally you want the motor trimmed neutral (level to the water, anytime you are angled up you are loosing efficiency) with a roost roughly the height of the cowling or a little lower. with the correct setback, weight distribution and prop/propwork its pretty attainable.

[lincoln72001]

TR4's are good as well..... add that to the list of props i PM'd you....BTW WELCOME TO SCREAMANDFLY!!!!!!!!!!!!!!


Thanks man, I've got just about everything out of the motor performance wise that I want, I was just wondering if a 4 blade would stick better and maybe give me some more speed, the boat is set up the best i could get it to run and i know I should be happy with it but you know you can always go faster.

[lincoln72001]

6" of set back, it's a deep hull so it's trimmed all the way up when it's on edge, the rooster is level with the cowling once it bites. the most weight is the gas tank which is centered and when the boat is on the top there's about the last foot of the transom in the water.