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Engine mounting or prop issue?

Old 09-24-2020, 08:57 PM
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Default Engine mounting or prop issue?

First, I am not sure I have an issue.. I recently purchased a 2007 Hydra Sport 2900 CC. It has twin Verado 300 XXL (30"). At WOT she gets to 5000-5150 RPM (49-51 MPH flat seas). I belive Max RPM Mercury spec for these motors is 5800 - 6400 RPM. I am propped with Rev 4 21 pitch props.

Cruising sweet spot is 4000 - 4100 RPM, about 34 mph at 1.4 burn. I get out of the hole and on plane quick and I am satisfied with top end speed, but something doesn't feel right all throughout the mid range. It feels like the i am slow getting to cruise speed and my RPMs are too high at cruise. Maybe its just in my head, but I am accustomed to hitting the sweet spot sooner. Maybe something like 30mph at 3500 RPM.

And since I can't come close to the max RPM at WOT, maybe my pitch is wrong or my engine height is off. Transom height is 38". Shaft height is 30". My anti -cav plates are 8" above the center keel, but right in line with the V. I feel like the engines need to be raised.


Any advice or suggestions are appreciated.



Old 09-24-2020, 09:01 PM
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Thats a heavy boat. 21 pitch 4 blades seems like way to much pitch. Your rpms are way low.
Old 09-24-2020, 09:08 PM
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Last edited by ocnslr; 09-24-2020 at 09:17 PM.
Old 09-24-2020, 09:09 PM
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Originally Posted by captain28570 View Post
Thats a heavy boat. 21 pitch 4 blades seems like way to much pitch. Your rpms are way low.
thanks. I forgot to mention they are 14. 5" x 21p.
Old 09-24-2020, 09:28 PM
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You around 13 percent slip. Not terrible but should be a lot better.

I would bet a set of 17 pitch Eco's or 17 pitch 4 blades would put you close on RPMS. Engine height would be easier to give advice on if you had a pic with the motors running. Showing where the plate is at cruising speed.
Old 09-24-2020, 09:54 PM
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The boat is slow, that’s the first thing that jumps out at me. The same boat with 250s can do 51-53 MPH, so with 300s I’d expect it to do 55-57 MPH. You need to find the lost speed, and I don’t think it’s simply the props, though they are a little too much pitch.

If the boat we’re going 55-57 MPH with those props, the WOT engine speed would still be low, so ultimately, you’ll end up with 20” or 19” pitch props.

The motors are clearly way too low. Their height relative to the keel is irrelevant because you have twins. You have to look at the surface of the hull bottom directly forward of the motors. With the significant setback you have, the motors will need to go up several inches. That will help.
Old 09-24-2020, 10:24 PM
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Thanks Tom... i gut feel is the motors are low. Are you saying the anti- cav plate should line up with the step up on the stern? If so, I am about 5" too low. I even wondered if this model boat would typically wear 25" shafts and perhaps the last owner put 30s on her at repower?

Anyone with a hydra 2900... what shaft length are you running?

Suppose I need to fix motor height before evaluating proo pitch?
Old 09-25-2020, 07:42 AM
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I agree with Tom. Looks like you have room to go all the way up. ......And find the other 1200 rpm. Also wondering about the same as you commented about the previous owner and 25/30" lowers installed. Keep us posted on your outcome.
Old 10-06-2020, 10:28 PM
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Bumping thread for follow up.

Can someone describe how to determine the mounting height heating engines on a deep V? I believe from the previous comments my props are too low. The Anti cavitation plate lines up with the V, though I don't know if that is correct mounting. I gave a little room to move the engines up, but wondering if this boat should be outfitted with 25" shafts instead.
Old 10-06-2020, 11:03 PM
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Did you play around with trimming the motors out to try to really get it up on the pad? Just because the motors are low didn’t mean they have the wrong shafts. Did you look and see with the motors can come up? As a previous poster said you need to see where the cavitation plate is while running the boat. I would not give up hope until you check a few things out.
Old 10-06-2020, 11:37 PM
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Originally Posted by tad117 View Post
Did you play around with trimming the motors out to try to really get it up on the pad? Just because the motors are low didn’t mean they have the wrong shafts. Did you look and see with the motors can come up? As a previous poster said you need to see where the cavitation plate is while running the boat. I would not give up hope until you check a few things out.

I've played with the trim plenty. As I previously mentioned, it doesn't feel right through the mid range. I believe I need to adjust prop pitch, but want to ensure proper motor height before I do. There must me a formula or the suggested height when hanging twins... hanging a single is widely publicized. I know where the anti-cav plate is when dry and running... just need to know where its supposed to be. That is the help I need.
Old 10-07-2020, 12:06 PM
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You indicate that you know where the cavitation plate is when you’re running but I didn’t see where you listed that. Maybe that would be a good indicator along with the low rpms. On a single, I read that the plate should be visible at cruise or wot. Wouldn’t the same criteria apply to twins?
Old 10-07-2020, 12:14 PM
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Originally Posted by Tiger51 View Post
You indicate that you know where the cavitation plate is when you’re running but I didn’t see where you listed that. Maybe that would be a good indicator along with the low rpms. On a single, I read that the plate should be visible at cruise or wot. Wouldn’t the same criteria apply to twins?
The anti cavitation plate is buried and not visible when cruising, though cruise RPMs are likely off. I believe the engines must be raised. I'd like to know a recommended height of the engines when mounted rather thank trial and error, especially since my prop pitch also needs to come down.

Essentially I am looking for a starting point for engine height... I'd like to think this is a question everyone a boat is repowered.
Old 10-07-2020, 12:37 PM
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I agree you are quite low, cavitation plate should be visible while at normal cruising speeds, skipping on top of the water.

Do not chase props right now, fix engine height, run it again.

my suggestion, go as high as you can, remember you have a 30 inch shaft to begin with, plenty of room to play.
Old 10-07-2020, 01:03 PM
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There are rules of thumb about how to approximate where the engines should be but that's just to get a better starting point for your trial and error process.

Generally as a starting point try to be 1" above the hull in line with the engine (the V) then add 1" for every 12" the the engine is back from the hull. Then you can trial and error from there.

If it was me I would just raise them as high as they can go and start there with the trial and error. You only have 3 holes to go up so probably not enough
Old 10-07-2020, 01:24 PM
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Thank you VTX. I do feel the engines line up in-line with the hull, but there has been no compensation for the distance between the motors and the hull. The step is probably 2'.
This also plays into my suspicion this boat (Hydra Sports 2900CC) should have 25" shaft length lower units rather than 30". Perhaps raising it the 3 holes will get me close enough...
Old 10-07-2020, 04:02 PM
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I would raise it up 3 holes run it then mess with the props 17-18 would be my guess.
Old 10-07-2020, 04:05 PM
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Raise those engines! Raised my 300 zukes 1 hole and it made a world of difference.

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