Question for the prop experts

**TraceF3** · 03-24-2018, 03:05 PM

I have a deep V offshore hull, 24'5" and about 5300 pounds including motor, fishing gear, and people.

I'm running a Merc 225 EFI, 1.75 gears I think. With a 17p Mirage Plus prop. It will run to the redline with a light load and works well for my application.

I'm moving up to a Opti 300 with a Torque Master gear case, also 1.75 gears. The Mirage Plus is well proven for single engine offshore boats so I'm going to start with that to see if it works as well with more motor.

Question is- going from a 225 EFI to a Opti 300... how much pitch should I go up? The Mirage Plus is made in 1' increments. Start with a 21 maybe?

**ghost28** · 03-24-2018, 04:01 PM

whats wide open speed and rpm with average load? what hull?

**TraceF3** · 03-24-2018, 04:42 PM

2003 Proline 23 Sport, the Donzi hull. WOT with average load 55-5600. Speed about 42-44 mph.

**Richard** · 03-25-2018, 09:41 AM

I'm not sure anyone can say with confidence right now. Put the new motor on an get a baseline with the existing prop. You might be ok with it. There are other options open to you. Depends on what you hope to accomplish.

**TraceF3** · 03-25-2018, 10:05 AM

I would. But the new motor is a CR so I have to buy something. I'm trying to get close.

Thanks!

**Rynobull** · 03-25-2018, 10:31 AM

What is the rev limit on the 225? I assume the 300xs you are going with is a 25" shaft and has the 5800 rev limit. Rev limits are probably similar.

Assuming 5,600 rpm with the 225hp and 17 pitch prop at 44mph, your slip is about 15%. Usually 50hp on hulls like that are good for 5 mph so 75hp more should be good for 7 to 8 mph more. So we can work backwards on our prop slip calculator and find that a 19 pitch prop at 5,800 rpm and at 14% slip (as the slip should decrease some with the increased speed) will give us the 51mph.

I would say start with the 19 pitch if you are happy running 50 or 51 mph but have a bit more punch and better weight carrying........or 20 pitch if you really care about top speed with a light load. I would be inclined to go to the 19 pitch and have better holeshot and load carrying in a fishing rig.

I think the 21 would be a bit too steep, especially with any load in the boat.

**staylor** · 03-29-2018, 11:02 AM

X2 on going to the 19 pitch Mirage Plus- the numbers I came up with was 49-50 mph with the 19 pitch. I think you'll just lose hole shot with a 21 pitch and gain only a little top end.
Doug

**TraceF3** · 03-29-2018, 11:23 AM

That's what I'm trying first. Found one "like new" on eBay for $125 shipped. So if it's not perfect at least I won't get hurt.

**CircleHook** · 03-29-2018, 11:51 AM

Originally Posted by TraceF3

I would. But the new motor is a CR so I have to buy something. I'm trying to get close.

Thanks!

New or new to you? If factory new in a single engine application - why CR?

**TraceF3** · 03-29-2018, 12:00 PM

New to me. A 2014 with 318 hours. $8800

**FUJIMO** · 03-29-2018, 12:00 PM

Noth'n wrong with a CR single installation. Let me think... Nope. Noth'n wrong.

.........

**TraceF3** · 03-29-2018, 12:01 PM

The boat isn't smart enough to know

**FUJIMO** · 03-29-2018, 12:05 PM

Oh wait a minute... Which handle do you throttle with? This could get tricky... Up, no...thats not a problem either.

**CircleHook** · 03-29-2018, 01:32 PM

Originally Posted by FUJIMO

Noth'n wrong with a CR single installation. Let me think... Nope. Noth'n wrong.

.........

I was of the impression that counter rotation lower units were not as strong as standard rotation lower units So if you are buying one new for a single application why would you buy the counter

**FUJIMO** · 03-29-2018, 06:27 PM

Originally Posted by CircleHook

I was of the impression that counter rotation lower units were not as strong as standard rotation lower units So if you are buying one new for a single application why would you buy the counter

This was sort of true back in the eighties when lefts first came to market, but have been improved a long time ago, thru design. If anything, it has been just the opposite regarding durability, for a long time now. Just one of many pieces of info out there that refuse to go away. Such as "toe" adjustment of twin gearcases, etc.