Planing Hull - Trim Tabs

[rangerscott275]

I had a quick question for some of those who are more technically savy...

I have a 53 classic MY powered with Cummins VT8-370's. (both rebuilt in 2003)...she is heavy and slow. I think the boat is most comfortable between 8 & 9 knots. the RPM for this (depending on current, etc.) is between 1500 and 1900 rpms. I think i burn around 15 gallons per hour at this usage for those interested. Also, the boat will do about 12.5 at 2200 rpm and burn close to 50 gallon per hour. I've never seen it go faster than that.

I figure I am limited by the hull design and overall weight of the boat...and the type of engines. But, my question is can this boat plane? Also wondering if trim tabs have much effect given speed and weight...I've noticed some difference when using them, but not too much.

Any other comments on the type of engine (as I don't know much about except that I was told they are very reliable) or other technical data welcomed! (for those of you who can't resist....I do have the manuals and plan to start reading!) thanks in advance!

 

[SKYCHENEY]

I tried to make an offer on a 1972 53MY with those same engines. The problem was that after I did all of the legwork and found all the answers as to the history of the engines from a guy that I know at Cummins, the broker bought it out from under me. Needless to say, I was not very happy. It was a beautiful boat and I hated to lose it after months of searching. Anyway, I don't know much about performance, since I never had a chance to seatrial her, but you are correct as to the reliability of those VT's.
As far as hull design--that boat will plane and it will do it with as little as 300-350hp per side but it does depend on load too. If she is weighted down with alot of personal gear, anchor chain, anchors, lead ballast, dinghy, outboard motor, etc., that will slow her down. You should, in theory, get about the same results as those with 8v71n's. Others can tell you what that would be.

 

[MikeP]

The hull will definitely plane - they are not displacement hulls. However, they are heavy so they need power! The 53MY with the oem natural DD 8V71s are generally considered not to be able to plane due to insufficient power to do so. Those with oem 8V71Tis will plane. Our 53MY with TIs will plane and, if lightly loaded, will get up to 18+ knots. Fully loaded, 15K is about it.

Your RPM for the speed are quite a bit higher than I observe our 53. Unless headwinds are quite brisk, 1500 rpm will deliver at least 10 knots. I'm sure others will respond with more info...

 

[rangerscott275]

I should note I have turbos on each as well.

 

[SKYCHENEY]

What is the rated hp on those rebuilds?

 

[rangerscott275]

I believe 350hp each...I have the engine survey / info at home and will try to dig up / post more specifics tonight for anyone interested. Just talking about the engines makes me want to start them up....I love the way they sound!

 

[Avenger]

Scott,

If you can get numbers off the nameplate on one of your fuel pumps I can look up your engine spec. The plate is supposed to be on top of the pump main housing, usually riveted right near the gasket for the front housing/ mounting flange. It's about 1/2 in. high by 2-1/4 in. wide. On a Cummins PT fuel system the "fuel pump" is also the governor, so if you're not familiar it's the thing that the throttle cable is attached to.

The info may also be on an engine data plate, but they are usually inaccessible. The fuel pump on a V-8 is right up top and easy to get to, the CPL number and fuel pump code from that tag will tell all. Of course, if it's on the survey and you don't have to crawl around the engine room I guess that would be easier.

 

[SeaEric]

Typical H.P. rating of the VT8-370M's is 370 HP. They are 785 cubic inch blocks, not to be confused with the VT903M's, which these could be as well. If memory serves me, those are the 370's in Scott's boat.

 

[rangerscott275]

Scrod,

the serial numbers are definitely VT8-370-M's...serial numbers are Port 633689 and starboard is 633688. Any info would be great! I don't have the engine survey (the elec version was on my laptop that got stolen..see "Crime" Thread, and can't find my hardcopy) otherwise I'd write up all the info it had in it for specs. Gotta call the surveyor and get another copy. Thanks again!

 

[Pascal]

i wonder why anyone would have repowered these boats with turbo engines developing not much more than the original natural and still unable to plane.

i get about 9kts at 1500 rpm depending on the water depth.... closer to 8kts in under 8' (even below 8kts in 6') and closer to 9.5kts in 12'+

that's with 8V71Ns... from previous threads, i think the Turbo'ed boats will be running around 1200/1300 rpm at 9kts... props are different to allow faster speeds.

 

[SeaEric]

I believe Scott's Cummins 370's are indeed originals, offered as an option to the 871N's in the early years.

 

[Nonchalant1]

You mentioned trim tabs in your title, and you do need them set right to get it to plane, particularly with lower power. I'd start with the tabs 3/4 down and run the engines at wide open throttle. My 53MY will climb up over the wave when it passes about 12 knots with almost full fuel and water (but with 435HP engines). If yours will plane, the bow comes down and you'll see the speed jump to 15 or better and you can back off on the throttle.

A comparison of RPMs with our other boats is probably nt meaningful because our props are pitched differently (mine are the original 28X31s factory spec for the 8V71TI engines). Lower HP boats have lower pitch props to match the engines.

If running WOT and 3/4 tab doesn't work, and your engines are putting out their rated HP, take careful note of the top RPMs your engines can reach at WOT under load. Then run them at WOT with no load (out of gear) and compare the top RPMs to the engine specs. If they're reaching top spec RPMs both ways, they're probably running fine. If they reach rated RPMs under load easily, you could add an inch of pitch to the props and try it again. It'll make you go faster, but it'll load up your engines more. They should still be able to reach rated RPMs under load, even if it takes a while, or they're overloaded.

Doug Shuman

 

[Avenger]

Scott,

I'd be glad to help. Per my earlier post, I need the numbers off the nameplates on the fuel pumps. If you have trouble finding them you can PM me with a phone number and a time to call when you're on the boat. I should be able to steer you to what you're looking for.