Off the shelf cam options for marine engines
02-15-2016, 09:38 AM
#161
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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.
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.
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02-15-2016, 09:38 AM
#162
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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...
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...
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!
02-15-2016, 09:47 AM
#163
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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 .
Last edited by Full Force; 02-15-2016 at 09:50 AM.
02-15-2016, 09:49 AM
#164
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02-15-2016, 09:51 AM
#165
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02-15-2016, 09:56 AM
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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.
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.
02-15-2016, 10:08 AM
#168
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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
02-15-2016, 10:27 AM
#169
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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.
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.
Last edited by CDShack; 02-15-2016 at 10:31 AM.
02-15-2016, 10:38 AM
#170
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