Offshoreonly.com - Off the shelf cam options for marine engines

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Quote:

Originally Posted by MILD THUNDER (Post 4404131)

I have a good amount of time invested in propping and running large offshore style boats.

I do agree that you need torque. However, i will not trade off upper rpm power for low end torque in most cases. What most fail to realize is we prop a boat so that the rpm at wot is where the engine makes peak power. If an engine makes 700hp at 6000rpm, in a particular boat, it might take say a 28p prop to get 6000rpm at full throttle. If the engine makes 600, or 650ft lbs at 3500rpm, the boat isnt going to have a faster top speed. It will accelerate faster, but most guys arent drag racing 35-40ft boats. Also, most of us, arent at full throttle at 3500rpm.

Ive seen way to many engines cammed like they are going in a drag boat or chevelle, that make great low end power, and fall on thier face in the upper rpm range, where things matter when your flogging it across the lake at full throttle trying to creep past your buddy.

My old setups had small roots blowers. Small overdriven roots can make lots of low end torque and response. Engines made as much, if not more torque with the smaller blowers. Switching to larger blowers, woke the engines up above 5000rpm big time. Boat is SIGNIFICANTLY faster . Does it go from 50-80mph as fast as it did with the smaller blowers? I dont know, never timed it, nor do i care about that. Im not drag racing an 11,000 lb boat.

So in you opinion, moving the torque peak higher in the rpm range would generally be a positive thing?

Quote:

Originally Posted by horsepower1 (Post 4404132)

Acceleration is ALWAYS an issue, whether you're drag racing or not. Top speed is one thing, but that "set you in the seat" thing will always impress me more. A good engine will have both.
If you and a friend have the same top speed but you get there quicker, your friend will never catch you. ;)
Not arguing, just sayin...

Yea, and 90 percent of boaters here, have outdrives that are already overpowered and cant handle hard acceleration constantly!

Im all for making good torque AND good peak power. As we know, sometimes that is a tradeoff .

Im referring to an offshore boat, that actually runs offshore. Not a lake runabout . Offshore guys battle it out for 30 plus miles in a straight line normally. Usually its on the gas, and it stays there , for a long time!

I would think so due to getting there faster if your boat will carry the power up top, I could be wrong but that's my guess .

Quote:

Originally Posted by bck (Post 4404135)

So in you opinion, moving the torque peak higher in the rpm range would generally be a positive thing?

Quote:

Originally Posted by bck (Post 4404135)

So in you opinion, moving the torque peak higher in the rpm range would generally be a positive thing?

Yes, within reason. If you raise it higher then the hp requirement of the propeller curve, you will never get to peak rpm power.

You think an extra 30ft lbs at 3400rpm is gonna help in this run? :D
https://youtu.be/tDh9V58xdg4

With the current setup peak tq has moved from 4600 to 5300

Quote:

Originally Posted by MILD THUNDER (Post 4404131)

In offshore application, the peak torque and peak HP should be around 1000 to 1400 rpm from peak to peak. For your 6000 rpm limit, I would want to see peak torque around 4600 to 5000 rpm. I would also like the peak numbers around 100 in difference. Meaning if you make 700HP then torque should be no less than 600. Ideally you want the number as close to each other numerically as you can get it. If you achieve this the acceleration will be off the hook. The torque number at Peak HP rpm number should be around 7% of the max torque number. An engine that does this will not only accelerate the hull out of the water but will run out to peak HP rpm and not hit a wall.

So what I gather is for the average boater an off the shelf cam AND/OR off the shelf cam lobe profile will be more then adequate. Even if the cam SPECS are custom the profile does not need to be expieremental

This way old school thought process over simplified, but it might still be relevant. I was taught drives generally have a design maximum, not just HP, but RPM. Relative to prop speed. All things have to be considered, because you can get caught up in the Dyno looking at numbers (guilty) and forget that motor on the stand is part of a system. In super ballpark numbers, most drives max out around 6000 rpm.

In "the compromise" You choose cams, heads, intakes etc. to pull the maximum torque/hp numbers around 6000 rpm. When you prop, you prop to max the motor out around 5800 rpm, then trim into 6000/6200. Torque down low isn't necessary for most of us, because like my surface drives, it takes 4000 out of the hole, and I can't even run on plane under 3000. Of course there are variations on the theme, but making 900hp at 8500rpm generally isn't as effective in a boat as a good torque number through your real operating range.

This was just the Rule of Thumb we used to use. May not viable anymore.

Quote:

Originally Posted by MILD THUNDER (Post 4404140)

You think an extra 30ft lbs at 3400rpm is gonna help in this run? :D
https://youtu.be/tDh9V58xdg4

Joe, I noticed in that video, he seemed to be WOT at a little over 5000rpm doing a little over 90mph. Do you think he is over prop;d ? Now I don't know where the torque curve of his motors are, or where max HP is. But what I am getting at is?? Wound it not be better on his motors if he were able to pull , say 6000rpm? And would he then also get more top speed? I have been on the assumption, that its hard on stuff when you prop a boat so it can't reach max rpm. Now I know nothing about his setup at all, just going on what it looked like in the video. .