fatcatone549
Member
Hi I have a 1999 215 express crusier.I replaced the 220 HP motor with a 325 Hp used the same outdrive ,and boat is slower with the new motor,could the problem be the prop? Dave
new larger motor boat goes slower
- Thread starter fatcatone549
- Start date
BillK2632
Well-Known Member
Both v8 or was the old motor a v6? If it was the out drive gear ratio is the problem.
In that year (1999) the 220HP would be a carb'd 5.0L.
You should still get the same exact speed out of the boat. The difference, though, is that you won't be able to use all of the power (you'll over-rev) without re-propping or changing the gear ratio. If you're losing speed, there's either something drastically wrong with the engine (not making full compression), maybe something like a dirty bottom, or there's user-error at play.
What type of motor did you put in?
EDIT:
Bill, for what it's worth, even if he originally had a V6, it wouldn't matter. The reason is that the new engine will still spin the outdrive up to the same revolutions (and, in essence, the prop) as the old engine did, which will mean the same top speed. Let's use the max RPM for the old motor as an example, 4,800RPM. The new motor will still turn 4,800RPM (which in leads to the same RPM's for the prop). But the difference here, is that the new motor will spin up to 4,800 with much less throttle input than the old motor, which will ultimately allow for the use of a taller prop. The taller prop will then allow for full throttle and increased speed.
But, along the gear ratio line of thinking, there could be an extreme case where there's just too much "new" HP that a prop, alone, won't solve the over-revving problem and a new gear ratio (and probably still a new prop) will be needed. However, even if he had this "extreme" case, he should still be able to get the same top speed out of the boat with the way things are now.
You should still get the same exact speed out of the boat. The difference, though, is that you won't be able to use all of the power (you'll over-rev) without re-propping or changing the gear ratio. If you're losing speed, there's either something drastically wrong with the engine (not making full compression), maybe something like a dirty bottom, or there's user-error at play.
What type of motor did you put in?
EDIT:
Bill, for what it's worth, even if he originally had a V6, it wouldn't matter. The reason is that the new engine will still spin the outdrive up to the same revolutions (and, in essence, the prop) as the old engine did, which will mean the same top speed. Let's use the max RPM for the old motor as an example, 4,800RPM. The new motor will still turn 4,800RPM (which in leads to the same RPM's for the prop). But the difference here, is that the new motor will spin up to 4,800 with much less throttle input than the old motor, which will ultimately allow for the use of a taller prop. The taller prop will then allow for full throttle and increased speed.
But, along the gear ratio line of thinking, there could be an extreme case where there's just too much "new" HP that a prop, alone, won't solve the over-revving problem and a new gear ratio (and probably still a new prop) will be needed. However, even if he had this "extreme" case, he should still be able to get the same top speed out of the boat with the way things are now.
Last edited:
BillK2632
Well-Known Member
Dennis - so curious why different gear ratios are recommended for different engine / hp? I know my 4.3 has a 1.81 and a v8 calls for a lower ratio. Never really dawned on me both turn roughly same rpm. Is it the power curve - i would think a 4 or 6 cylinder would reach higher rpm with less throttle than a 8 cyl?
[FONT=Arial, Helvetica, sans-serif]Alpha One Gen II
[/FONT]
[FONT=Arial, Helvetica, sans-serif]Alpha One Gen II
[/FONT]
[FONT=Times New Roman,Georgia,Times]Horsepower[/FONT] | [FONT=Times New Roman,Georgia,Times]Engine[/FONT] | [FONT=Times New Roman,Georgia,Times]Year[/FONT] | [FONT=Times New Roman,Georgia,Times]Ratio[/FONT] |
| 120 & 140 | 4 cylinder | 1991 thru present | 1.93 / 20-24 |
| 175 & 4.3 | V-6 | 1991 thru present | 1.81 / 17-19 |
| 198 | V-8 | 1991 thru present | 1.62 / 24-24 |
| 198 thru 260 | | 1991 thru present | 1.47 / 20-22 |
I'm not sure I can completely explain it, but think of it in car terms. With cars, we have multiple gears that we can shift through to stay in the "power band". What's one of the main reasons that newer cars have a higher number of gears? Performance and efficiency. With multiple gears, the engine can stay in it's happy place where it provides the best combination of power and efficiency, along with having even lower upper gears to allow lower RPM's on the highway (again, more efficiency along with decreased noise). With boats, we have only one gear so other things have to change to find a happy medium between HP, Tq, expected load (weight of boat), prop size, altitude, etc. So a combination of gear ratio and prop size must be selected to give optimum performance.
Your V6 engine spins 1.8 times for every 1 revolution of the prop. The higher the ratio is, the greater the mechanical advantage that is given the engine. Given the same application, lower HP engines will need a greater mechanical advantage to perform the same work. Prop pitch is another variable that comes into play, which of course, just makes things a little more confusing. But, generally speaking, high HP = taller (higher) prop. Lower HP = shorter prop -- but, if you go too short, you limit your speed so increasing gear ratio (greater mechanical advantage) will allow for a taller prop.
I hope that makes sense...
Your V6 engine spins 1.8 times for every 1 revolution of the prop. The higher the ratio is, the greater the mechanical advantage that is given the engine. Given the same application, lower HP engines will need a greater mechanical advantage to perform the same work. Prop pitch is another variable that comes into play, which of course, just makes things a little more confusing. But, generally speaking, high HP = taller (higher) prop. Lower HP = shorter prop -- but, if you go too short, you limit your speed so increasing gear ratio (greater mechanical advantage) will allow for a taller prop.
I hope that makes sense...
jthatcher
Member
Nail on the head Dennis. Very clear, straight forward breakdown.
BillK2632
Well-Known Member
Thanks Dennis, I always thought the gear ration was related to rpm, not torque/power - good explanation, I learned something today.
Last edited:
Bt Doctur
Well-Known Member
Guys, I use a 25 ft I/O that had a 350 260hp motor 1.50 R , we had to install a 220hp 5.0 and using the same drive we could only get 3200 rpm out of the motor. We switched to a 1.84R and can now turn 4400.
Going the other way "up in hp" would have the opposite effect. too much rpm with the same prop and either cavitate or run with a reduced throttle setting =reduced rpm.
Going the other way "up in hp" would have the opposite effect. too much rpm with the same prop and either cavitate or run with a reduced throttle setting =reduced rpm.
fatcatone549
Member
I had a V8 220 HP merc now have 275 HP merc
Last edited:
dpmulvey
Active Member
- Dec 19, 2007
- 4,188
- Boat Info
- 1999/2007 330 Sundancer Hardtop.
- Engines
- 6.2 Merc Horixons
1.5:1 Velvet Drives
Sounds like he went from a 5.0 to a 5.7. Is their any weight difference?
scoflaw
Well-Known Member
- Aug 10, 2011
- 6,395
- Boat Info
- 1999 Powerquest legend 260 sx 502 mag
2005 Baja 242 islander 496
- Engines
- 502 mpi Bravo 1
496 mag B1
Old thread, I'm sure a taller prop got him where he wanted to go
JVM225
Well-Known Member
The 5.0 and 5.7 are the same block with a different bore and stroke. There should be no real difference in weight.
If the new engine with it's higher HP is not achieving the results the lower HP, and presumably more tired, older engine did. Then I would think the first place to look for a problem would be with the new engine itself, or possibly something related to the installation with regards to the ignition or fuel systems.
Assuming that the new negine is good:
Was this a complete engine, or a long block installation?
Was it brand new, or rebuilt?
People refer to lots of things as new engines.
Did the installer use the same intake and carb? If so, there may be a problem there.
Did the installler use the same ignition system? If so, is the ignition timing coming in properly? I'm not a marine mechanic, and not sure which ignition system that era Sea Ray had, but my 87 had the Thunderbolt IV ignition control modules on it's old motors. There were different ones for different motors. Not all V8's were the same. Each had a unique point at which total timing came in.
If the timing is not right, that would effect performance.
If the new engine with it's higher HP is not achieving the results the lower HP, and presumably more tired, older engine did. Then I would think the first place to look for a problem would be with the new engine itself, or possibly something related to the installation with regards to the ignition or fuel systems.
Assuming that the new negine is good:
Was this a complete engine, or a long block installation?
Was it brand new, or rebuilt?
People refer to lots of things as new engines.
Did the installer use the same intake and carb? If so, there may be a problem there.
Did the installler use the same ignition system? If so, is the ignition timing coming in properly? I'm not a marine mechanic, and not sure which ignition system that era Sea Ray had, but my 87 had the Thunderbolt IV ignition control modules on it's old motors. There were different ones for different motors. Not all V8's were the same. Each had a unique point at which total timing came in.
If the timing is not right, that would effect performance.
- Nov 12, 2006
- 5,334
- Boat Info
- 1999 Sea Ray 330DA
Twin 7.4 MPI (310 propshaft HP) V-drives
- Engines
- Twin Mercury Marine marinized 7.4L L-29 V8s
Cast iron block w 4-bolt mains
The reason why more powerful engines have less reduction ratio, especially on the Alpha drive, is to reduce torque multiplication. More reduction ratio is generally a good thing as the prop then can turn slower, which reduces drag while allowing a prop of full diameter and standard pitch values. With ratio closer to 1:1 rather than closer to 2:1 the pitch would have to be very shallow, which is terribly inefficient.
More powerful engines need less reduction ratio in order to properly load the prop without a lot of pitch that's also inefficient. But more importantly, since the reduction ratio is on the top gear set, more reduction, that is, more torque multiplication, would apply so much torque to the lower gear set and dog clutches that the service life of the drive would be greatly reduced.
More powerful engines need less reduction ratio in order to properly load the prop without a lot of pitch that's also inefficient. But more importantly, since the reduction ratio is on the top gear set, more reduction, that is, more torque multiplication, would apply so much torque to the lower gear set and dog clutches that the service life of the drive would be greatly reduced.
