Power, Torque, RPM, Speed question

[Mahoney]

I wanted to ask a question relating to RPM and speed as it relates to the power and torque output of the engine.<br /><br />All things being equal, prop trim, boat wieght, atmospheric conditions, water surface. If your engine is able to turn 4000RPM and achieve 35 MPH. is it possible that increasing the horsepower and or torque would possibly reduce prop slip and get you more speed at the same RPM?<br /><br />The reason I ask is I recently discovered my advance was not advancing much more than 20 deg, and have fixed that so it advances a full 32 deg above 3500RPM. While the engine this year had always sounded and ran great, I also disovered through checking the plugs, one of 8 cylinders was never firing due to a small carbon track in the dist cap. This has now been fixed. All 8 Cyls pounding and timing correct.<br /><br />With this added power output from the engine, will the boat be moving faster at the same RPM, or simply be able to achieve a higher RPM?

 

[chuckz]

Re: Power, Torque, RPM, Speed question

At the same RPM it will run the same speed. However, it should get to that RPM faster and have power to turn more RPM's and thus go faster.

 

[Limited-Time]

Re: Power, Torque, RPM, Speed question

Mahoney,<br />Once you have established your engine is tuned and running correctly. You need to find out what your WOT RPM range should be, (4000 seems low to me but I am no expert) then find a prop that puts you there. You may have to work with a prop shop. I am sure some one here can get you headed in the right direction. Good Luck.

 

[Mahoney]

Re: Power, Torque, RPM, Speed question

I am currently still in that process, the previous prop on the boat was whoafully underproped and would spin well up past 5000. This is an old OMC engine and WOT is 4000-4400RPM, the new 14X17 prop with a couple people on board was able to flirt with 4200 even with the 7 cylinders and retarded advanced timing.<br /><br />Not sure if the 2 issues fixed this week will be enough to warrant a 18 or 19 pitch prop or not. If with 2 large people on board it achieves anything over 4500, I might prop up an inch.<br /><br />I am checking it out this weekend to see what I have.

 

[LubeDude]

Re: Power, Torque, RPM, Speed question

If you were running at the max RPM (according to your manual), before the repairs and if after the repairs you have more HP, You may be able to use a higher pitch prop which would give you posibly more speed. But it appears you were running slightly under your recommended max RPM, (probably due to the poor performance of your engine), so now you will likely be running a higher RPM and that should give you more speed.

 

[Scaaty]

Re: Power, Torque, RPM, Speed question

Basically you will just have the throttle positon backed off a bit. The other 7 cylinders had to work a little harder to make up for the 8th. And just like a car motor on the highway, once at speed, you are barely working the motor. <br />And this is... "will the boat be moving faster at the same RPM" ..impossible with out changing a prop pitch

 

[QC]

Re: Power, Torque, RPM, Speed question

Originally posted by Robby6950:<br />And just like a car motor on the highway, once at speed, you are barely working the motor. <br />

Way wrong dude. The engine stops gaining RPM at WOT on a marine engine precisely becasue it is working as hard as it possibly can with the available horsepower equalling the required horsepower to keep the propeller turning at that speed. Car yes (kinda) boat no. Absolutely full load. All she's got, guts and feathers, pedal to the metal, pouring the coal to her, wick all the way up etc. etc. etc. <br /><br />Robby, your statement is progressively more correct as speed drops. A marine propeller load curve is basically linear, i.e. load goes down as RPM drops (engine and prop RPM drop together), so when you slow down, the engine's output (bhp) goes down. Torque rise doesn't enter into the equation except to jump on plane. Torque is always there, but it drops with RPM. bhp=torque x RPM/5252, so if RPM drops and the load is linear, both torque required and bhp created, drop.

 

[Bondo]

Re: Power, Torque, RPM, Speed question

Ayuh,........... Gotta Agree with the Cat,.........<br /><br />Think of a Propeller as a Dynomometer...........<br /><br />I Also Agree that Reguardless of HorsePower,.......RPMs = MPH in a Boat..... The Dyno thing again.......

 

[QC]

Re: Power, Torque, RPM, Speed question

Bondo's all over this one. A propeller is almost a perfect dyno. We used to do weed-eater fuel system development (yes, I am serious. The smog police in CA will pay for anything ), way too small for our dynos, so we used a big model airplane propeller to load it. Worked great.

 

[Doug Durako]

Re: Power, Torque, RPM, Speed question

Mahoney----you should hit 40 mph at 4000 rpm.<br />Everything else is trim, weight and hull design, and with that heavy Cobalt and old OMC drive, you lose on the weight and trim options. Those drives were designed to run deep and that Cobalt hull weighs 1000 pounds more than modern 19-foot runabouts.<br /><br />In 1974, 45 mph in a heavy family runabout was really fast.

 

[Mahoney]

Re: Power, Torque, RPM, Speed question

yah, I realize I am not going to be a speedster, even with a 225 hp V8. You take the good with the bad. Big heavy, sturdy hull. Deep V for plowing through rougher water, but not a speed deamon for sure.

 

[Scaaty]

Re: Power, Torque, RPM, Speed question

Alright, a retract on the auto comment..forgot the torque converter equiv and not thinking of a locked tranny...BUT. QC, you are going into theory of engines. And I'm pretty sure that the Torque/HP equal at 5252 is pretty irrelevent to 99% of most boat engines. <br />The question...<br />"If your engine is able to turn 4000RPM and achieve 35 MPH. is it possible that increasing the horsepower and or torque would possibly reduce prop slip and get you more speed at the same RPM?"..... My thing here is you could put a gazillion hp at the prop......same rpm is same rpm. Ya just might get there a little faster. And nowhere on the original post did I see WOT mentioned (QC-Way wrong dude. The engine stops gaining RPM at WOT ).....again, the question was.."All things being equal, prop trim, boat wieght, atmospheric conditions, water surface. If your engine is able to turn 4000RPM and achieve 35 MPH. is it possible that increasing the horsepower and or torque would possibly reduce prop slip and get you more speed at the same RPM? ...I'm sticking with my "the throttle will be backed off a bit. Same as losing a sparkplug, will make ya push harder on the throttle. And in fact, it would be the same in a car. IF you ARE at WOT, well, we all know the answer. Just trying to not re-write the books here, just keep it simple, and answer the original question, not discuss theory. No biggie...<br />

 

[QC]

Re: Power, Torque, RPM, Speed question

Robby,<br /><br />Sorry. Not trying to get into theory. My only point was that marine engines don't loaf "at speed" just adding the back up for anybody that is interested. I think I am starting to see your point and it is supported by one of my points. If he has more horsepower than he does now, if all other things are equal (RPM etc), he will be at less throttle and the same speed. I think we were saying similar things. My only beef was the "you are barely working the motor" thing.<br /><br />BTW, the bhp=torque x RPM/5252 is not theory. It is valid for every engine type and every engine application. It is just the calculation that is used to quantify horsepower. Without it a manufacturer could claim anything he wanted. There has to be an equalizer. Without it Merc would say "my engine is rated 320 bhp" and Volvo would say, "not it isn't, it's a 298", Merc would say "no, today I feel like it might be a 343, so there" and on and on.<br /><br />Also, I get carried away with this topic, it is one of my favorites. When I finally understood it, it was like an awakening . . . Brothers and sisters that equation will set you free.

 

[Scaaty]

Re: Power, Torque, RPM, Speed question

Like I said, bad anology on my part. We tend to get a little deep here on simple questions, so I just try and keep it simple too. And we are on the same wavelenght, just a little differently explained. And 5252 had better be known by anybody who post an answer here, or its back to school. Anyway, heres a good one fer ya. On a open motorcycle, who's faster at WOT, skinny little kid, or a 280 pound fat guy?

 

[QC]

Re: Power, Torque, RPM, Speed question

Originally posted by Robby6950:<br /> On a open motorcycle, who's faster at WOT, skinny little kid, or a 280 pound fat guy?

No wind, no grade? This sounds like an aerodynamics puzzle, so I'm gona go with the fat guy. Uphill I'm going with the kid . . .