Right engine size for 6.2m Sea Nymph Commander?

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    Posted: 23 Apr 2022 at 5:33pm
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The old engine on my 6.2m Sea Nymph Commander is a 140HP 2 stroke Yamaha. It's time to replace it, and I'm wondering what size of engine would be right? While I don't have a wide variety of experience I get the impression it's a heavier than average boat.

Yamaha have new 130HP and 150HP options. I don't want to underpower it, nor weigh it down and shake it to bits with something too big. Any recommendations?

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Post Options Post Options   Likes (0) Likes(0)   Quote smudge Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2022 at 6:04pm
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Where are you fishing David? 150hp would probably suit most situations but if you're crossing bars or heading out wide I'd be looking at 200hp
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Steps, come on in!
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Post Options Post Options   Likes (0) Likes(0)   Quote davidc Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2022 at 6:06pm
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Wellington harbour, across to Kapiti Island on a nice day, and other sheltered waters. Nothing rough.

Would 130HP be sufficient, or really under powering it?

Cheers.
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I'm not an expert but I think 130hp would be underpowered. My boat lacks hp and it really restricts my fishing. Given the choice I'd go for more power every time
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Originally posted by smudge smudge wrote:

Where are you fishing David? 150hp would probably suit most situations but if you're crossing bars or heading out wide I'd be looking at 200hp
200 might be a bit big mate, quite a few around with 115 which I think would be min, 130hp would be mid and 150 would be getting up there.
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Post Options Post Options   Likes (0) Likes(0)   Quote davidc Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2022 at 8:12pm
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Originally posted by Kandrew Kandrew wrote:

200 might be a bit big mate, quite a few around with 115 which I think would be min, 130hp would be mid and 150 would be getting up there.

That's for the 6.2m commander? I'd be happy with a mid size engine.

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Originally posted by davidc davidc wrote:

Originally posted by Kandrew Kandrew wrote:

200 might be a bit big mate, quite a few around with 115 which I think would be min, 130hp would be mid and 150 would be getting up there.


That's for the 6.2m commander? I'd be happy with a mid size engine.

Sorry didn’t have my glasses on, thought you had the smaller commander, smudge is right 130 to small 150 would be ok but a 200 would give good power.
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Post Options Post Options   Likes (0) Likes(0)   Quote shaneg Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2022 at 11:40pm
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Would of thought 150 hp or up would be what you would look at for heavier 6.2 mtr hull. Recommendation is based on common sense rather then indepth theoretical calculations, as in don’t put to big a donk on… as lot of people doing that, they tend to ride like pigs in any but ideal conditions including crashing a little harbour chop. Get in a real sea then it all about balanced weight distribution for any hull… big weight in arse with horse power you can’t actually use then becomes a liability. All depends what your boat is needed to do. If racing on flat water go for biggest donk and max horsepower.

Offshore as in game-fishing, or crossing blue water or longer passages with swell,  go for balanced horsepower …as in enough to maintain decent speed without as much weight in arse. Sure some might diagree but 150 hp four strokes are big motors and would be plenty on back of yours.
Very familiar with my own 16 ft stabi.. 60 fourstroke is plenty and very economical. Also have a mate 635 Bonito I fish on a bit has 150 Honda and goes real well. 
Don’t over power it.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 24 Apr 2022 at 10:42am
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OK long post, but has the experience, the reasons why and hows that I would have like to know when going thru this exercise... and assume others have the same thoughts/ questions

1st the Commander is 5.5m or 18' 2"
And they are an awesome salt water hulls at rest and on the plane, but like all hulls, cars, tractors , trucks to be any good for the end use, need the correct power.
 
With about 80L fuel under foot, guys ave around 90kg, fishing gear, 95L bin, bait burly bin, spare props anchors, battery etc andwith full bimini up.. Its always up.. comes in around the 1250kg total gross weight on water.

 We originally had 115 hp on the back fishing the Bottom end waikeke, Corro furth, Ahh ahhs etc
 Good but min  good power  WoT around the 40 mph cruise 32/34 mph around the 3900/ 4000 rpms.
Get in bit ave chop would cruise thru well, and run on an oilly rag. Chop get up a little more and then start to work the throttle and once that starts, gas goes up huge.

 Then where thinking about moving home, to West coast. Had time as next few months was hard out me doing complete reverbish the old old house and gardens etc. I had crossed bars many times over the yrs, and knew the 115 if something happens like wave climbs in front etc, or need to move quick, need that bottom end power.
Along with  crotch straps and actual life jackets..
I worked out (calculated) and also being on several other commanders and similar hulls that confirmed the equations around the min 130 to 140 hp would be the go.

Now came the 20" transom...and looking for a later model , good service record, 140hp with 20" leg.

 I was just tossing up to re build transom, but is still solid as, so thinking make up a simple jack plate or a full plate across the transom... cost to buy is stupid price.

Eventually a  V6 150 20" came up , ex lake , low hrs, serviced winterised from new.
 Loading the engine bay with the 115, Extra weight would set the stern down at anchor about 1/2 to 1" and nil diff once boat moving.

Swapped engines was a breeze, over and done in couple 3 or 4 hrs and 1 other person to hold transom bolts from inside... and engine hoist.

Result :
Get in serious chop on top of westcoast  swell or  a good westerly down the bottom of  Tamaki straight, white caps at near shoulder height, set throttle and chug home just on the plane and not falling off it... off Bottom end Waiheke  good stherly comes up and tide changes..

Have had 1 instance in the commander a wave stand up in front on the bar...got to it just as started to break, over, then there is always the next last swell of the set, big period, get to the next before started to break.....the 115 the 1st would have broken, have the power to power thru.. I dont know. I did the 2nd would have been on us, and bigger..
Couple instances of breaking bar waves coming in from the side going in.. yes the 115 would have done it fine, the 150  instance move lot heaps area and time around us.

Now cruise at the old cruise speed of 32/34 mph fuel consumption drops WELL over 10% better economy.. If travel at around 3900 rpms and 38/39 mph very similar as previous 115  32/34 mph economy....
WoT is around the 48/49 mph now as gone from a 80L (which is all that fits in the std commander 120L tank due to air vents) to a full 150L fuel tank... Take lighter guys, big fish bins out, 1/2 tank fuel goes around the 52 mph ...starts to chine walk a little... not my cuppa tea.

Down side, well only 1... With tthe 150, I have to say every time we take off, " Holding  on?"  And always turn and check everyone is.. there is always some 'hero' who wants to fall backwards.. Its put into gear, boat moving, move throttle gently forward a little... wait to go over the bow wake start trim up, and now at cruise speed.. no throttle adjustment.

It is my understanding that these boats in general, made around the 70s /80s where rated to around 140/150 hp..

So 2 stroke 115hp good for general harbour stuff.  4 stroke 120/130 will do your bars , good chop etc.

The 115 and the 150 are/ where both correct height within 1/2 adjustment notch (bars on drops slightly) propped to correct WoT and Cruise slips, and pitch..



 
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Post Options Post Options   Likes (0) Likes(0)   Quote davidc Quote  Post ReplyReply Direct Link To This Post Posted: 24 Apr 2022 at 1:50pm
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Thanks again Steps. There is definitely a shortage of good second hand 20" shaft engines out there.

If I understand it sounds like you're saying 130HP 4 stroke should be fine?

Looks like the 130HP Yamaha is the same 1832CC as the 115HP engine, but gets more HP out of it.

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If I understand it sounds like you're saying 130HP 4 stroke should be fine?

Rather not just "fine" but very nicely powered..

Looks like the 130HP Yamaha is the same 1832CC as the 115HP engine, but gets more HP out of it.

Cant comment on that as not familiar with yamaha...but yes.. most makes of lots stuff, cars, lawn mowers etc have same cubes with different hp. Personally Im old school  a fan of the cubes. But going on data I have collected over the yrs this doesnt have any significant effect unless the boat tends to be low or under powered.

Calculating what prop as a close if not close enough, based on the gross weight of our Commander on the water, the 115 and 150hp numbers and think the  yammy  gearbox ratio is 2.15:1 and max rpm range 5300 to 6300 (please confirm these) around a 14 to 14 1/4 X 19 or 20 pitch.
 
As to stainless or alloy, on data collected off our boat, any distortion of blades starts to have an insignificant effect around the fast cruise and doesnt become significant till around the 42 mph.

I use an alloy prop mainly because if clip something in shallow water




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Post Options Post Options   Likes (0) Likes(0)   Quote Kandrew Quote  Post ReplyReply Direct Link To This Post Posted: 25 Apr 2022 at 9:17am
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So he’s already got a 140hp on the 6.2 metre boat and your saying to go down to a 130hp motor.
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Post Options Post Options   Likes (0) Likes(0)   Quote davidc Quote  Post ReplyReply Direct Link To This Post Posted: 25 Apr 2022 at 9:44am
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And the old engine (a 1990 Yamaha) is a 2 stroke and the new would be 4 stroke.

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Still 140hp at the prop and 130hp at the prop, the old 2S 140 will be better out of the hole.

Outbourds are very expensive and most of us can only afford to do it once, fit the wrong motor and your stuck with it. I would spend that little bit extra and go with the 150hp at least, you won’t regret it and I would go S/S prop at the same time.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 25 Apr 2022 at 10:11am
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So he’s already got a 140hp on the 6.2 metre boat and your saying to go down to a 130hp motor.

 Which poses the question.. is the hull a Sea nymph 6.2 hustler or a 5.5 Commander?
 I have made it clear my comments are for a 5.5m Commander

Also of note is the issue of 20" transom mentioned by the poster.. which is the Commander. The 6.2 is 25" on pod.

 If the hull is the longer and heavier 6.2 Sea Nymph, then I would be suggesting going at least 175 hp, preferably 200 for bars
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Post Options Post Options   Likes (0) Likes(0)   Quote Kandrew Quote  Post ReplyReply Direct Link To This Post Posted: 25 Apr 2022 at 10:23am
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His question was in the above post

If I understand it sounds like you're saying 130HP 4 stroke should be fine?

Your answer was

Rather not just "fine" but very nicely powered..

But yes the first question is it a 6.2 metre boat needs to be confirmed.
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Post Options Post Options   Likes (0) Likes(0)   Quote davidc Quote  Post ReplyReply Direct Link To This Post Posted: 25 Apr 2022 at 6:18pm
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I was told it was a 6.2m Commander by the seller (about a year ago), but it never occurred to me to actually measure it myself. Maybe I should do so! If it's really 6.2m then maybe I'll go for the 150HP, otherwise for a 5.5m perhaps a 130HP would be fine.

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Post Options Post Options   Likes (1) Likes(1)   Quote davidc Quote  Post ReplyReply Direct Link To This Post Posted: 25 Apr 2022 at 6:24pm
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Originally posted by Kandrew Kandrew wrote:

 I would go S/S prop at the same time.

Should I specifically ask for a stainless steel prop? I hadn't really thought about it. Presumably these are the best?

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Originally posted by davidc davidc wrote:

I was told it was a 6.2m Commander by the seller (about a year ago), but it never occurred to me to actually measure it myself. Maybe I should do so! If it's really 6.2m then maybe I'll go for the 150HP, otherwise for a 5.5m perhaps a 130HP would be fine.

Have you got some photos you can post up, as steps says the commander was 5.5. We can tell by the photos.
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