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Re-powered SeaVee 340B with 2018 350 Verados

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Re-powered SeaVee 340B with 2018 350 Verados

Old 05-06-2019, 10:06 PM
  #1  
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Default Re-powered SeaVee 340B with 2018 350 Verados

Any Input on this subject would be much appreciated.

I recently acquired a 2004 Sea Vee 340B that needed some TLC. My biggest upgrade was re-powering the Boat with 2018 350 V-Rods. The engines I replaced were 275 V-Rods and they had a 30 inch shaft. On these new 350s with the 25 inch HD lower units I mounted them in the highest hole on the engine mount to the transom and the diffrence in the depth of the prop is not that much with 25 inch shafts vs the 30 inch so I decided to go ahead and give it a shot with the 25 inch shafts and maybe be able to save some money on the extension kits to convert to 30 inches. I got the engines for a good deal so i didnt mind buying 25 inch shafts. Long story short the boat seems to be running well and does not spin out on fast sharp turns nor does it sound like it is getting air while cruising. What i am questioning though is the fact that my cruise speeds have not seemed to change much from the 275's and when i look at guys in THT posting cruising vs rpm numbers I seem to be off about 3 to 5 mph across the board. For example several guys with the same boat engine and props are saying 4000 rpms 30 mph and 4500 35 mph full of fuel and loaded to fish. I am only seeing 4000 rpms 25-26 mph and 4500 rpms 29-30 mph. BTW the boat revs to 6300 RPM which is very close to the factory recommended 6400 running revo 4 17 pitch props

Do you think that the shorter shafts can have something to do with this? Maybe they are not grabbing enough water ? Does anyone recommend getting the extension kit and for what reason ?
Should I expect a huge difference in speed over the 275's or just more power/torque getting to the same speed ?

I am not a speed demon I just want to Cruise at 3800-4000 RPMs and not fall off of plane and according to guys with the same setup i should be able to do so.

Thank you in advance for any input

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Old 05-06-2019, 11:16 PM
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i had the same issue with my contender when i switched props. i was on mirage plus 19, went to rev 4 17 and then to labbed 3 blade tempest plus. you will not loose rpm and pick up speed. rev 4 are vey good mild cruise with heavier seas and do not vent. personallly i would have the revs 4 labbed and add some cup so u pick up a little speed
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Old 05-07-2019, 03:02 AM
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Where is the cavitation plate when you’re up and running? Have you thought about trying different props, like Ecos?

This is a good question for THT member sundancekid. Joey works for SeaVee’s service dept and is the THT go-to for SeaVee questions.
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Old 05-07-2019, 04:55 AM
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Originally Posted by davidwademarine View Post
Where is the cavitation plate when you’re up and running? Have you thought about trying different props, like Ecos?

This is a good question for THT member sundancekid. Joey works for SeaVee’s service dept and is the THT go-to for SeaVee questions.
He knows a little about Mercs, too.

OP, I’m really surprised the 25s work if 30s were original. Have you calculated your current prop slip?
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Old 05-07-2019, 05:19 AM
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Try a set of Merc ECO props if you want better cruise numbers. I found that the Rev4 is a good prop for running at high speeds but slips at slower/cruise speeds compared to the ECO.
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Old 05-07-2019, 12:02 PM
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I am going to take the boat out shortly I will attempt to post pictures of where the cavitation plates are when the boat is on plane. Im just wondering weather this can be caused from the shorter shafts or if this would be more of a prop situation. I will try to get pictures as well as exact numbers of RPM vs cruise speed. Hopefully Sundancekid gives his opinion. I was surprised the 25s worked as well but I can do full speed hard turns and they do not break loose however maybe somehow they are what is causing my issue.
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Old 06-19-2019, 05:35 AM
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Did you ever end up switching props?
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Old 06-22-2019, 04:35 PM
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I did not end up switching props. After talking to a couple of prop guys and goin through some ruff weather I realized that I do need to switch to 30 inch shafts due to the props catching air when the bow goes down in heavier seas. If my cruise numbers are not where I want them after I change the prop shafts and re finish the bottom this week then i will experiment with the Eco 3 blades and the the bravo FS 22 which is what Sea vee recommends. I will post an update.
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