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RPM's HP's ETC.

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josh

Well-known member
Just wanted to introduce my self and ask a few questions. I run a 15 x 7 Air Ranger with a 2:1 belt , 330hp 350 and a 2 blade powershift.

My question is how are some of you getting such low RPM planing and cruising speeds.

I have been following some of your disscussions about low gear ratio's and bigger props which I am starting to understand, but is this possible on a boat like mine. Or is this only possible on small light hulls with big horsepower? I have tried to add pitch to my setup, which does lower RPM's, but it seems to just make the boat lathargic.

Is there something I'm missing or is it just a hp to weight ratio thing.

Thanks

Josh
 
kinda answered you own question there josh it's all about rpm torque = hp by the way thats a pretty good set up you have there we have two of them at work 330hp 2:1 belt drives with 2blade power shift propsone diamond back the other gto ,gto runs the hill the best diamond back is faster the both cruise at around 3000 rpm in deep water with no head wind most heavy boats do ,maximum rpm around 5000 at 55 mph with light load. All props being about equal in the amount of thrust for the amount of hp applied and all boats needing a certain amount of thrust to propel it at a certain speed ,you either haved to add hp and more prop or lighten the boat. Just my opinion
 
hey josh, good to see a fellow texan on here. i'm in the process of putting my air ranger together right now. neways, one thing you need to consider is that those guys are slinging 3 blades with 600 hp on light hulls. you're sling 2 blades with 330 hp on a heavier hull. one way i think of it (and i might be wrong) but consider this formula. lbs of thrust per lb. of boat. multiply 4 times 330 (4 lbs of thrust per hp is the 'ideal' but not realistic figure). although it has been done. and divide the weight of your hull by this number. this shows a huge difference in thrust per pound. simply put, your rpms will have to be way higher because you don't have the power to weight ratio they have. they are also running 2.68 : 1 reductions that allows them to build more torque than the 2 : 1. btw, where are you located?
 
Josh, you have another option too. Your stock 330hp, 350 is a great little motor but the lobe seperation on the cam is around 112 deg. You can install a cam with less lobe seperation (around 108 deg.) which will bump up your torque, and then spin a 3 blade prop that is a little smaller in diameter.

I run a 350 also .... I went from a 76" two blade to a 72" 3 blade, and my motor is puny by most small block standards.... probably around 300hp. Throw some roller rockers at it for a couple hundred bucks ( good for 15-20hp) and you'll be good to go.

BF
 
Thanks for the advice on the cam and roller rockers.

Duckluv,
I run in matagorda and Rockport. I beleive you know some freinds of mine, Jason and Greg?

I am familiar with the basic theories of power, tourque and thrust. The thing I was wanting to know is if it is possible for the COMMON AIRBOATER (1500# boat with 300-400hp's) to be able to cruise at 2000 RPM and plane at 1200. Or is this only possible on a 500# step hull with 600+ hp?

What kind of hp/reduction would be required to run a 2 blade version of THunders prop?

Josh
 
A friend of mine has a 330HP G.M. crate motor with a 2.68 ratio Rotator on a 15ft hamant hull he cruises at 2,200RPM. In my experiences the 3 main things that dictate what RPM you plan and cruise at is (1) the hulls running bottom, (2) prop (3) weight. I have seen 900 plus horsepower boats with 4 blades all the way to ten blades require 3,000RPM just to plane, as always it's the entire combination not just one thing. I have seen 14ft 450HP aluminum head small blocks with belt drives take 3,500RPM just to cruise. Everybody always seams to think something is the most important it's the whole package. You need to pick out what you want your boat to be the best at and go from there. Right now personally I'm going for fuel economy. My hull is very free and fast and fuel efficient but I what to go ridding all day on ten gallons now I have my boat cruising at 1,900RPM's with a 2.68 ratio gearbox if I had a 2to1 gear box I couldn't come close to this it would be more like 2,500RPM's.
 
That's what I am trying to do is figure out exactly what I want to put together.

I mostly run fully loaded 40 miles round trip to my cabin. There is some open deep water crossing. The LS1 appeals to me because of the weight reduction, but I don't know much about them or there limitations. Keep in mind this is all salt water running with a lot of spray hitting the motor. Of course low RPM's would be nice, but is it possible to load the prop up to keep the RPM's down and still have the ability to run dry fully loaded?

With my current rig I can pitch it to cruise at 3000 or less, but I lose a lot on the top end if I do.

It sure seems that a variable pitch shift on the fly prop would solve a lot of our problems.

josh
 
Josh, a variable pitch (constant speed) prop would run you well over $50,000, and would be something else to break.
Thunder, Cntry and a few others are creepin' up on the perfect combination. It sounds to me like you and I are up against the same problem with cruise power ...... we just need more of it :? .

BF
 
Actually its just a few days now til Ill be landing back in the real world. Anxious to run around and meet some ya'll and see your rides. I may or may not buy a ride this time in but we shall see.

Scotty
 
Something to keep in mind:

HP = (torque x RPM)/5252.

What that means is that HP is really just a number. The real beef is, how much torque does the engine have at each specific RPM.

Just to say "my motor has 330 HP" doesn't tell the whole story. You need to know at what RPM does it have 330 HP (or whatever HP)

The more torque you can make at low RPM, the more prop your engine will be able to turn, and the less RPM you will need.

A gear box or belt drive multiplies the torque of the engine by the ratio it turns the prop at. It has the same effect as making more torque in the engine except it also requires a higher engine speed for the same given prop speed

The more prop you are able to turn at one given RPM, the more performance you will get from your boat.
 
A gearbox develops more torque at any RPM and horsepower is just as significant as torque you measure one to get the other. Talking about only torque is like saying ya got a 3/4 race cam or a 30-30 cam.
 
josh, i know your buddies. greg and i talk on the phone pretty often about airboat, hunting and "VINTAGE"......lol. (inside joke). thats a good group of guys. anyways, when i set my prop up on my 0540 initially at 3050 rpms. it would run dry ground like all get out but my cruising rpms were like 2200. i prefer to keep them lower. now i have it set at 2850. runs dry just as good as 3050 but i have great power at low and high rpms. now i cruise at 1800. this is 200 rpms less than my wooden prop with a lot more push. TAKE HOME MESSAGE: find the happy medium with what you have. i'm sure you can get better results with a different gear box and prop, but you have to weigh in the cost factor.
 
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