Trojan Boat Forum

Well Hello All, been awhile since I last posted, finally got rid of the Chrysler 400 & repowered with a Marine Power 5.7L MPI, installation went OK, re-built the velvet 1:1 ratio trans, all is good as far as that goes. Now I'm faced with prop issues again! I'm hoping that someone can help, I bought a new Mich Wheel 14/12 which only gave me WOT of 2400 RPM, re-pitched it to a 14/10, gave me 2600 RPM WOT, cut it down to a 13/10 w/minor cup, got me to 3000 RPM / WOT I need to be 4600-5200 RPM WOT.
Current Conditions Are:
Boat 1976 F-30 Express
Weight 8100 lbs Dry ( I Think ) if anyone knows please let me know?
Planing Hull
Trans 72C Velvet Drive 1:1 ratio
26.5 ft in water at water line
Draft 2.5 ft to bottom of rudder
At 3000 RPM running the 13/10 WOT speed is 27.6 mph, need to lose some speed, gain some torque, don't want to go to a smaller dia prop would like to cover as much of the rudder as posible for control at low speeds. At $600 a pop I'm thinking of going back to 14" but maybe a 4 pitch.
Any Help or advice is greatly appreciated
Thanks All, Abster

With the horsepower of the 5.7, I think you're on the right track using a 14" prop. The 13" wont have enough surface area for the horsepower you're throwing at it. A #4 pitch however sounds a little low in my opinion. I'd be thinking maybe in the #7 pitch range with a cup.

Also are you sure that there isn't something else holding your boat back? Is the bottom clean and are the tabs working correctly?

27 mph sounds pretty respectable for a single screw boat but at 3000rpm?

An F30 with a single ??? Ouch!!!!

What prop did you have on with the Chrysler 400, and what speed / rpm were you getting out of that? If the MPI is delivering similar HP and torque as the 400 was originally designed for, the same or very similar factory spec prop should probably be the right one (as I am sure you know, the prop doesn't care what colour the engine is painted - just the oomph coming from it).

You say you need to lose some speed and gain torque... what makes you think that? Does she lag getting out of the hole and trying to get on plane? If she's just on plane and doing about 10-12 mph at 2000 (real) RPM, that's about right. You should be able to accelerate from 1500 RPM / 8 knots to 2000 rpm/just planing in about 4-5 seconds. If you can do that, torque seems about right to me for the hull.

Your top speed seems about right to me too (especially given the single engine) so unless you have serious lag problems, I would suspect calibration of the tach is just throwing your thinking off... check with a multimeter and see if the tach is just messed up.

For what it's worth : an F32 with twin Chrysler 360's runs a 16x16 cupped prop. If I remember correctly, I think my father's 28' Sea Skiff (built in 1969, wood hull, single Chrysler 383) ran a 12x13 prop. You should be somewhere in between, but perhaps closer to the 28' specs...

Thanks Paul, Hull is repainted & Glossy, Tabs work fine, Exactly 27MPH is Great for this single but it should be @ 4800 RPM not 3K. This is definetley a dia / pitch problem.

Thanks Maniac, The prop that came with the boat was totaly wrong it was a 14/17, the 400B Chry was tired and old, the repower included a new dash & guages which are accurate, what I'm afraid of by not acheiving the power curve of the motor ( 3200-4600 RPM crusing RPM) this can cause increased cylinder head temp, increased fuel consumption not to mention additional stresses on the trans & shaft components, everything is new from water/sep, anti/siphon, cutlass bearing, ect. have not missed anything, hull is done with an AWL grip anti-foul paint, ethenol free fuel, ect....Does any one know for sure what the dry weight of the F-30 Express is? 8100 lb is supposedly the spec on it?

Thanks, Abster

What is the hp of the engine ? I take it it is in 300 -350 hp range ? with a 1 to 1 gear you need to be able to spin that prop at 4600-5000 ( same as the engine , a 14" prop is to much as you would need to a 6 or 7" pitch way to flat IMO ( if they even make on that size ) a 12x10 prop would be close to getting you the rpm you need . give your HP and I will run the numbers for you .


The stock prop this boat came with won't even be close as that engine got its HP at a much lower rpm .

8100 lbs should be close

Thanks Prowler,

The rated HP is 335 at the Crank @ 5200 RPM
If it does need a 12/10 that will be great, I can cut this one ( 13/10 ) down again to a 12 the problem is I can't re-pitch it any lower ( already been pitched down from 12 to 10.

Thanks, Abster

What's the torque? Boats need torque.

BOATDIESEL.com has the best prop calc - $25/year ...

There really is no way you could be getting 27.6mph today - the math doesn't work:

3000RPM x 10pitch @ 1:1 =with 0% slip (impossible) is 28.4mph.
3,000 RPM
x 10 In/Rev (pitch)
/ 1 :1 Gear Ratio
= 30,000 Inches/Min
/ 12 ft/Inch
= 2,500 ft/min
x 60 min/Hr
= 150,000 ft/Hour
/ 5,280 Ft/Mile
= 28.41 MPH


Your tach or your speedo are way off ...

12X10 no cup would not hurt to check the tach also .

I am not sure how a 14x12 cut down to a 12x10 will do .But a true 12x10 will put you right where you should be , I suspect a cut down and repiched prop will not work as well as the blade shape is not what it was made to be . ( love to see a photo of the cut down wheel) did they reshape the blades ?

Bite the bullet and get a 12x10 your engine will thank you .

Six pack says I am right

BTW I am talking 3 blade

IMO a 12" prop does not have sufficient surface area to handle the horsepower. Have you submitted info for a prop analysis?
http://www.miwheel.com

Paul wrote:IMO a 12" prop does not have sufficient surface area to handle the horsepower. Have you submitted info for a prop analysis?
http://www.miwheel.com

Paul I ran a prop analysis and it comes up recc a 12" prop . Remember this is a 5000 rpm engine and a 1 to 1 gear . we are dealing with a high shaft speed .

If I enter a 14 " diameter it comes up with a 6.5 pitch prop . a little too flat . I believe the lowest pitch for for a 14" you can get is 10 .

Yes using a bigger wheel would be nice but to do that you would need a reduction gear .

If money was no issue then I would recommend a reduction gear and larger prop and of course a bigger shaft. but he has what he has , and thats what we are working with .

Thank you Paul & Prowler,

Prowler did you run a 13" dia? I agree and would like to run the largest dia 3 blade also. Not looking for speed so much as I am looking for Fuel Efficiency and the least amount of stress / work placed on the engine ( A Happy Area ) rarely will this boat be run at WOT but would like a nice cruising speed whatever that ends up being.

You guys are Great! Thanks for all the help
Abster

PS: I will probably purchase a new 12/10 LH if a 13 won't work. What do you think about a new 12/8 or 13/? then I can tweeek it up or down 2 if needed?
Again Thanks

Abster et al,

Always good to start with good data.

"Trans 72C Velvet Drive 1:1 ratio
At 3000 RPM running the 13/10 WOT speed is 27.6 mph"

is not possible. So you have erroneous tach, prop or GPS data to begin with.

GIGO

I just ran it with 13" and it comes up with 13 X 8.5 so a 13X8 or a 13X9 . The may make a 13 X 9 , the min. pitch is normally .7 of diameter from what I have seen .

I still lean to the 12x10 but you could go with the 13x9 and get into the lower part of the needed rpm range ( or close) remeber NO CUP , if you add cup you need less pitch .

One thing that comes up is "Increasing the transmission ratio will improve prop efficiency." and that is true but not a cheap way to go

rossjo wrote:Abster et al,

Always good to start with good data.

"Trans 72C Velvet Drive 1:1 ratio
At 3000 RPM running the 13/10 WOT speed is 27.6 mph"

is not possible. So you have erroneous tach, prop or GPS data to begin with.

GIGO

Hard to say WHY ??

I say that becuse a 14" prop cut down to a 13" Is not like 13" wheel not even close , I would not trust the prop shops numbers is this case anyway , A good shop would have talked him out of cutting it down .

How ever I do belive the Tach should be checked for sure

You can get a 14" prop in a 9" pitch. That's what I have on my boat. This would still not be a low enough pitch to get you into your desired RPM range. I'm no expert on props however I did some research on the subject tried several configurations on my single screw before finding the right fit. What I found was that horse power and your ability to apply it to the water is directly related to the surface area of the prop (E.A.R.) Ultimately a 14" would probably best suit your engine however as stated earlier it doesn't look like it would work out for you. I don't think that I would stray to far from this size though. You'll probably find a low enough pitch prop in a 13" diameter to get you where you need to be and as Prowlersfish stated, "no cup". During my testing of different props, I found that pitch had a greater effect on rpm than did the diameter based on 13" & 14" props.

One more note on "cup". A cupped prop not only affects the forward motion but it also helps prevent cavitation. If after running an un-cupped prop for a while you notice some pitting on the prop, it may be due to cavitation. I slight cup may reduce this effect.

Hope this helps.