What motor size can i use ?

[cadeco]

Hi guys !

I bought a superfisherman 20' 1983 an it has a 150 hp Johnson ( 25" shaft). Recently someone offered me a 200 Mercury w/ a 20" shaft. Right now the boat has a cut out transom and i will close it to have a bracket installed. Question would be any problem to use the 20" motor ? I have read that many times when installing brackets, the owner would have to raise the motor, so tht's why i'm asking.
Thanks for the answers.

[OilFieldMan]

Your motor length will depend on the setback of the bracket. It's an open ended question with too many variables.

For a baseline I'll give you specs of someone I know's rebuild. He had a 25" shaft motor, installed a hermco bracket and needed a 30" shaft motor to keep from blowing out when turning. There is a rule of thumb that for every foot the motor moves back you need to drop the motor.

[Bushwacker]

Originally Posted by Yankee29

Your motor length will depend on the setback of the bracket. It's an open ended question with too many variables.

For a baseline I'll give you specs of someone I know's rebuild. He had a 25" shaft motor, installed a hermco bracket and needed a 30" shaft motor to keep from blowing out when turning. There is a rule of thumb that for every foot the motor moves back you need to drop the motor.

You're right that there are a lot of variables and one of the biggest is the prop! In general a prop without much cup in it will tend to "blow out" easier than one with a lot of cup.

The rule of thumb that's often quoted is that you have to RAISE the motor an inch for every foot of setback, because the water flows upward after it clears the transom. The size of the boat and the speed you run may affect that. Both the BRP and Verado web sites say that optimum motor height is when the AV plate is above green water. Most dealers seem to mount the motors too low, I suspect because inexperienced operators will complain if the motor "blows out", even though that may be due to operator error (trimming motor too high), but if they mount the motor too low, there is less chance of blowout complaints, and most folks aren't savvy enough to realize they're losing performance due to excessive drag from a too-low motor!

In my experience with a 30" Hermco bracket on a 20, the 1"/foot-of-setback rule is not nearly enough! Attached pic shows the bottom of my AV plate at 3.25" above keel, AFTER I raised motor 2 holes (as high as possible) from where Don Herman originally mounted it. With motor at this height, the AV plate was completely underwater, so I had Don redrill the motor mount holes to raise it another inch, but AV plate is STILL under water when I'm up on plane at about 25 kts and fully trimmed out! Don't have any problems with blowout if running at or below 50% trim, where prop shaft is parallel to keel, but I'm running a good PowerTech 15x15 4B prop with a fair amount of cup. Bottom line is that the AV plate on my 25" shaft motor is now about 4.25" above keel, and it still needs to be raised at least another 1/2", so I would say there is a reasonable chance that a 20" motor would work if you mounted it as low possible and run a good cupped 4B prop! Worst case might be that you'd have to buy a Bay Mfg. extension kit if it didn't work out, so might be worth taking a chance! Only downside is that your powerhead will be closer to the water than mine. The E-TEC cowl is very watertight and air intake is at extreme top of cowl at the back, so I wouldn't worry too much about an E-TEC but I don't know how the Merc cowls and air intake compare.

[cadeco]

Originally Posted by Yankee29

Your motor length will depend on the setback of the bracket. It's an open ended question with too many variables.

For a baseline I'll give you specs of someone I know's rebuild. He had a 25" shaft motor, installed a hermco bracket and needed a 30" shaft motor to keep from blowing out when turning. There is a rule of thumb that for every foot the motor moves back you need to drop the motor.

Thanks yankee29.

I believe i read raise, not lower and that's the reason i was/am contemplating that motor.

[cadeco]

Thanks Bushwacker.

That's what i was thinking , but, if the rule is " 1 inch for each foot" then , the 20" would not work because using it i would need a 24/26 inches setback giving me 2", maybe 3" to be raised and because the difference between shafts is 5" i would be 3 inches too short. What do you think?
thanks again...

[hermco]

Get a bracket designed for a 20" shaft motor. The bracket geometry/design determines the shaft length of the motor. Most are designed for 25" shaft length motors. You might be able to use some brackets designed for 25" shaft motors with a 20" shaft motor if you can mount the bracket low enough. Depends on the brackets design.

[Bushwacker]

Originally Posted by cadeco

. . . That's what i was thinking , but, if the rule is " 1 inch for each foot" then , the 20" would not work because using it i would need a 24/26 inches setback giving me 2", maybe 3" to be raised and because the difference between shafts is 5" i would be 3 inches too short. What do you think?
thanks again...

My point was that I think the 1"/foot of setback "rule" is bogus, at least for a Hermco bracket on a 20' SeaCraft Seafari! In my case the AV plate needs to be somewhere around 4.5-5" above keel for a 30" setback. My old motor mounted on transom had the AV plate about 1" above keel. So I needed to raise motor about 3.5-4" higher when moving motor aft by 2.5'. Therefore the "rule" in my case would raise motor 3.5" to 4"/2.5' = 1.4 to 1.6"/foot of setback!

Before anyone casts this "rule" in concrete, let me double check my AV plate height, as I haven't actually laid a ruler on it since Don raised my motor approximately 1"! Will try to get a measurement/picture in the morning!

[cadeco]

Originally Posted by hermco

Get a bracket designed for a 20" shaft motor. The bracket geometry/design determines the shaft length of the motor. Most are designed for 25" shaft length motors. You might be able to use some brackets designed for 25" shaft motors with a 20" shaft motor if you can mount the bracket low enough. Depends on the brackets design.

Thanks for the input, but the idea is this : Since using a 25" motor on a normal ( made for a 25 ") bracket, the 25" motor must be raised, using a 20" motor on that bracket would be ok and the 20" would not need to be raised-- correct?--
If i use a normal 25" bracket today and latter i change the motor , the only thing to do would be to raise that (new 25") motor instead of buying a new bracket. If i have to use a 20" bracket there will be no deal!--Someone says to install a extension (if does not work), but the cost of the extension will offset any gains on the 20" motor's price.
Again thanks! think about and tell me where i'm wrong ( if i am).

[Bushwacker]

I rechecked current AV plate height today and it's 3.75" above keel, not quite as high as I thought. The pic I got isn't real clear (couldn't find my old white plastic scale); the metric side of the scale is actually more readable than the english units . . . the 9.5" cm measurement converts to about 3.75". The AV plate is still underwater when on plane; my guess is that it needs to be raised to at least 4.0 to 4.25" via a jackplate or spacer. The middle pic was taken before Don redrilled the mounting holes to raise the motor and last pic was taken after raising. The most obvious difference is the position of the lower bolt relative to the 2" tall bootstripe.

My 25" motor is currently mounted as high as it will go. I need to check hole spacing on motor bracket to determine total height adjustment available, but I suspect a 20" shaft motor might actually work if you mount it as low as possible with bracket mounted at height Don recommends, and with mount bolts positioned in top holes so you can raise it if necessary. although you will have to decide if you can live with the powerhead relatively close to the water!

[dave s]

Although not SeaCrafts, my buddy and I have 25' Contenders, and have a similar situation as you.
We both run a single 225 Yamaha four stroke on the original double floatation bracket setup for twin motors; didn't move the brackets.
Mine is a 30" leg and his is 25"; I had to raise my motor as high as it'd go and even added a small jackplate- raised it two inches and the AV plate is still buried. I can raise it even more.
His motor is as low as it'll go but it will cavitate occasionally, usually when it's rough or if he trims up too much.

If you stick with the 20" motor, it'll probably work, but you may want to lower the bracket also.
If you going to repower with a 25" later, then set the bracket higher now.