Appropriate HP outboard for '65 Starcraft Mustang?

[John Schinnerer]

Hi all, I'm new to the forum, recently acquired a mostly restored 1965 Starcraft Mustang runabout. I am going to be buying a modern engine for it and I'm wondering what experience anyone can share to help me decide what HP to put on it for my purposes.
Per spec sheets I found elsewhere on this forum, it is 515 lbs empty, 5 passengers/1100 lbs max load, and rated for 45 HP.
The max I am considering is 30 HP, for several reasons. It came with a vintage '62 Johnson 28 HP which runs unevenly but when it kicks in solid seems to be plenty HP (and at that age it may not even be putting out 28 HP any more...). Also anything over 30 HP in modern 4-stroke is more weight in the stern than I want.
All the generic ballpark formulas I find say 1 HP per 25-40 lbs of boat, but they don't say anything about different hull types, or other factors like loaded weight vs. empty weight.
Main uses will be cruising/exploring local lakes with family/friends, and some fishing. Most often just 1-3 people in the boat, not a full load. Not planning on towing any skiers or toys on any regular basis, if ever.
So I am thinking either a 20 HP or a 30 HP. My guesstimate is, the 20 HP will be adequate but nothing much in reserve, and the 30 HP will be plenty even with a full load (better power to weight ratio, lower throttle for same power output). On the other hand, a 30 HP weighs 25-35% more (depending on make), and all that is on the transom, and is going to burn more fuel overall even if it does better at high speed than the 20 HP due to lower RPMs. So maybe 30 HP is overkill for my needs. My bias tends towards keep it small light and simple which pushes me towards the 20 HP but I don't want to end up underpowered and regretting it.
Let me know your experience with similar situations...thanks!

 

[racerone]

The 62 model 28 HP is a wonderful motor.-----Easy / cheap to maintain.----Some parts are easier to find than much newer motors.-----It has the universal magneto that was installed for about 40 years.---A dual ignition system.----Should top system fail it will run ( not fast ) on the bottom cylinder.-----Parts for that magneto are every where and cheap.-----Lower unit gears and bearings are bigger more robust than newer motors.----They use more fuel than a modern motor.-----But have you priced a modern 30 HP motor ?

 

[matt167]

That’s going to be a short shaft, so 25hp is the limit in modern times. That is basically an old 30hp. You can get a Suzuki DF25A and put the DF30A ECU in it along with the 30A intake and that will get you a short shaft 30hp for around $6k

 

[John Schinnerer]

That’s going to be a short shaft, so 25hp is the limit in modern times. That is basically an old 30hp. You can get a Suzuki DF25A and put the DF30A ECU in it along with the 30A intake and that will get you a short shaft 30hp for around $6k

Thanks, and, it's not a short shaft. Mustang has 21" transom, per spec and per my measuring tape - the transom on this one was notched down about 1 1/2 inches in the center, I think because the old Johnson is neither short nor long shaft by modern standard but an inch or two shy of long. Or maybe it was notched prior to that motor for some other reason, I don't know. I would fill the notch if/as needed to fit a modern long shaft. Might be fine as is.

 

[racerone]

The 1962 28 HP came in 1 version for a 15" transom.----A 5" spacer could be installed to make it a long shaft for 20" transom.----Does your motor have the 5" spacer ?

 

[John Schinnerer]

The 62 model 28 HP is a wonderful motor.-----Easy / cheap to maintain.----Some parts are easier to find than much newer motors.-----It has the universal magneto that was installed for about 40 years.---A dual ignition system.----Should top system fail it will run ( not fast ) on the bottom cylinder.-----Parts for that magneto are every where and cheap.-----Lower unit gears and bearings are bigger more robust than newer motors.----They use more fuel than a modern motor.-----But have you priced a modern 30 HP motor ?

Thanks, that's good to know, it will be good ad copy for selling it...ha ha...I bought this boat to use, in the water, not to fuss with an old motor, much as I appreciate vintage mechanical stuff of various kinds (I'm a vintage air-cooled VW fan for example, done everything with a few of those, including full engine rebuild). But in this case, been there done that, and done with that!
The motor did have lower end service, new gear oil, new reeds, some carb work, etc. just prior to me buying the rig. And I checked ahead and found as you say most parts are easy to find and not expensive. The guy I bought it from sent me videos of it running, before I went to see it (was an hour+ away). Then he got water in the engine housing, cleaning the rig it to make it look nice for me. It wouldn't even start for a few days (water in the ignition most likely). He couldn't get it running fully right again after that, and he spent a lot of time fussing with it, pulled the flywheel to clean/dry the ignition, messed with the carb, etc. I offered a reduced price based on the engine situation and it was accepted. I then did some more tuning and fussing myself, but could not get it fully happy either, in the time I was willing to spend on it. I had already decided I would probably replace with modern motor within a year or two anyhow...so yeah, I'm aware of the prices!

 

[John Schinnerer]

The 1962 28 HP came in 1 version for a 15" transom.----A 5" spacer could be installed to make it a long shaft for 20" transom.----Does your motor have the 5" spacer ?

Model # on motor is RX-10C which matches '62 Johnson 28 hp per multiple sources (just to verify that). I've attached some pics of lower end. No spacer - would be part #56 here, would be real obvious.
You can see the notch in transom - it's 1 3/4" down from original height. And the actual measurement from mount point to cavitation plate on my motor is about 18 1/2" - which is about halfway between short and long, compared to same measurement on some modern motors (ex. Tohatsu 30hp is 16.75" for short, 21.75" for long, & 20hp is 16.3" for short, 22.1" for long).
And, still wondering about my original question...modern 20 hp enough, or go with 30 hp?

 

[John Schinnerer]

That’s going to be a short shaft, so 25hp is the limit in modern times. That is basically an old 30hp. You can get a Suzuki DF25A and put the DF30A ECU in it along with the 30A intake and that will get you a short shaft 30hp for around $6k

When you say a modern 25hp is basically an old 30hp - you mean the modern 4-stroke EFI 25hp is going to have power comparable to an old two-stroke carbureted 30hp? Or 28hp as my case may be?

 

[racerone]

If money is no objecct buy a new short shaft motor.

 

[racerone]

Measuring of motors to determine if it is long or short has been confusing for quite some time.

 

[matt167]

The spec sheet from Fiberglassics does indeed say 21”. And yes, prior to 1986ish for OMC, horsepower was crank rated. After 1986 was prop rated.. the difference in rating led to about a 5hp loss on the same engines. You could get a DF30A and have it basically like an old 35hp. But 4 stroke isn’t as snappy. It’s quieter and no smoke however. I run a 2020 DF25A on my 1966 Holiday 14

 

[redneck joe]

I vote stay with the old motor. So much less to go wrong with it.

 

[Scott Danforth]

I would run the old motor and sort out the rest of the boat. Then after a year, come back around to the motor

 

[racerone]

most of my newest parts motors a 4 strokes.------Too expensive to fix .-----owners ruining them.------Shops not interested in fixing them.-----No mehanics available in short season to fix them.-----pick your reason.-----Looked at some newer parts motor 25 hp V-2 cylinder 4 strokes.-----I want one just to see how it is put together.

 

[IslandExplorer]

I'd go with the 30 new or stay with the old reliable, cheap, easy to fix motor. The 20 might be fine cruising around slow but it will be expensive for what it is and it will be heavy for just 20hp. Older boats assumed lower transom weight for the HP (bc of bone simple 2 strokes) - often overlooked. More weight can be just too much sometimes or it can *sometimes* be mostly compensated by more HP and battery and fuel moved forward, etc. So like most things, it's a comparison of trade offs.

My exhaustive take on the situation:
Runabouts should be light, nimble, and with a shallow draft.

-New 20hp = heavier than the 2 stroke and lots less power and liveliness- I'd call this one the frugal minded but "expensive and slow" option.

-New 30hp = way heavier than the 2 stroke, but a touch more power.. Would prob feel about the same power as the lighter, shaft HP rated 28. "Expensive and fast" but with a fat ass..

-Mint old 28 2 stroke. *Even all new CDI ignition, water pump, fuel lines, starter, etc. installed by a marine mechanic would be far cheaper than a brand new motor- the prices are absolutely criminal! Maybe buy one from a reputable shop that has legitimately been rebuilt. Keep the original as a spare. = "Cheap and fast" with normal weight balance.

You can always bite the bullet and buy a brand new motor down the line, even at a moments notice typically, assuming money isn't an issue. I would honestly trust a mint condition older motor like that more than a new one. minor things are often fixable enough on the water to get you home.

 

[IslandExplorer]

Had a 16' SS bow rider with an old Merc 40hp on it. It did -OK- but the motor was always working pretty hard. The old (but well maintained) 1988 70hp 2 stroke I swapped on never even broke a sweat and cruising fuel economy actually improved. A year later I put a brand new ignition system in myself for a couple hundred and it was even better, like a brand new motor but without the added weight.

 

[John Schinnerer]

I would run the old motor and sort out the rest of the boat. Then after a year, come back around to the motor

Thanks, that was my original plan if the old motor was solid enough to use, but it's not. The boat itself is mostly sorted out and ready to go. There's small stuff I'd fiddle with or improve but it's 100% usable, while the motor isn't.

 

[John Schinnerer]

I'd go with the 30 new or stay with the old reliable, cheap, easy to fix motor. The 20 might be fine cruising around slow but it will be expensive for what it is and it will be heavy for just 20hp. Older boats assumed lower transom weight for the HP (bc of bone simple 2 strokes) - often overlooked. More weight can be just too much sometimes or it can *sometimes* be mostly compensated by more HP and battery and fuel moved forward, etc. So like most things, it's a comparison of trade offs.

My exhaustive take on the situation:
Runabouts should be light, nimble, and with a shallow draft.

-New 20hp = heavier than the 2 stroke and lots less power and liveliness- I'd call this one the frugal minded but "expensive and slow" option.

-New 30hp = way heavier than the 2 stroke, but a touch more power.. Would prob feel about the same power as the lighter, shaft HP rated 28. "Expensive and fast" but with a fat ass..

-Mint old 28 2 stroke. *Even all new CDI ignition, water pump, fuel lines, starter, etc. installed by a marine mechanic would be far cheaper than a brand new motor- the prices are absolutely criminal! Maybe buy one from a reputable shop that has legitimately been rebuilt. Keep the original as a spare. = "Cheap and fast" with normal weight balance.

You can always bite the bullet and buy a brand new motor down the line, even at a moments notice typically, assuming money isn't an issue. I would honestly trust a mint condition older motor like that more than a new one. minor things are often fixable enough on the water to get you home.

Appreciate your thinking thru this. Agree with light, nimble runabout, that's why I was thinking maybe a modern 20hp would do. I've already looked at weight comparisons. The '62 Johnson 28hp is spec'd at ~127 lbs (mine has electric start so ~4-5 lbs heavier if the spec is for manual start). Lightest current 20hp 4-strokes are 97-110 lbs depending on options (Tohatsu, Suzuki). The lightest 30hp (Tohatsu) would be within 5-10 lbs of the old Johnson (depending on options). So no huge weight penalty for 30hp.
And yeah more HP per $$ with the 30 vs. 20 for sure.
Battery is already set up to be forward by previous owner, FWIW.

 

[IslandExplorer]

Well definitely let us know which way you go with it and it'd be cool to see a picture of the setup when she's ready. Could definitely get a few bucks out of that old motor to nicely discount the 20, or 100% guilt free justify the 30! lol Battery already up front is great.
I was thinking the weight of the 4 strokes were going to be much more dramatic of a difference, at that size that's not bad at all. Even for the 30- within 5-10 lbs is negligible on your size boat. Seems the 4 stroke vs 2 stroke weight penalties only become really significant on the 70+ HP size ranges, power trim models, etc.

 

[John Schinnerer]

Well definitely let us know which way you go with it and it'd be cool to see a picture of the setup when she's ready. Could definitely get a few bucks out of that old motor to nicely discount the 20, or 100% guilt free justify the 30! lol Battery already up front is great.
I was thinking the weight of the 4 strokes were going to be much more dramatic of a difference, at that size that's not bad at all. Even for the 30- within 5-10 lbs is negligible on your size boat. Seems the 4 stroke vs 2 stroke weight penalties only become really significant on the 70+ HP size ranges, power trim models, etc.

I will do that - at this point it's going to be the 30hp is my thinking. More hp per $$ and per pound, and if I want a slightly bigger boat at some point it can transfer over as there's a bit of power to spare.
Some of the 4-strokes are much heavier even in the smaller sizes..Yamahas are the heaviest by far, though they change carcass at 25hp while the rest change at 20hp then 30hp. Tohatsu lightest, Suzuki next lightest. And of course if you want power tilt/trim that adds a couple handfuls more lbs. to any make.
I'll post a couple current pics - it will look the same except for a different motor...