TOPIC: motor size for a 1959 Glastron redfish

I have a 1959 Redfish, but the motor is a 1983 VRO 70 hp Johnson. I purchased this boat recently, and so am learning all about her. It seems to me that the engine is not the right fit for this boat. The previous owner put sandbags up under the bow, and it's a bear to get her planed out right, very sluggish until you find her sweet spot, plus even with this big engine, she only goes about 25 mph. My only point of reference was my dad's 1957 Redfish which seemed to plane out perfectly and quickly, with a smaller engine - and of course, no sandbags. Any suggestions about what I can do to get her fitted with the right size engine for her size/ and or modifications with this engine to fit the boat better

No intention to offend by this question,...
Any chance the 70HP is a 20" mounted on a 15" transom?
doc

Like the good Doc mentioned. If that lower unit is hanging too low you'll have to raise the transom or find a short shaft.
Now on my personal end.....70hp is a good starting point. How about a 135hp on a 15 footer.
As the last resort you can raise the transom like I do.

Another possibility is your prop.
Get the numbers off your prop to find out if it’s a 15/13 or what ever and try going down a size or two. It would help a lot if you have a tac so you can tell if you engine is running the right rpms. But sounds like to big of prop for this boat.

you can calculate the correct horsepower rating easily. You need only three numbers.
Centerline length
Maximum transom width
Transom height (cutout to the bottom of the transom)
Multiply length times transom width . The multiple is the factor.

If the factor is greater than 52 AND the boat has remote steering (a steering wheel ) AND the transom height is 19 inches or more (a 20 inch transom) the HP rating = Factor times 2 Minus 90 = HP Round up to nearest 5 HP

If the factor is greater than 52 AND the boat has remote steering (a steering wheel ) AND the transom height is less than 19 inches (usually around 15) the Factor times 0.8 Minus 25 = HP Round up to nearest 5 HP

see newboatbuilders.com/pages/hp.html

The answer you get will probably be the size of engine your Dad had on his boat because the formulas haven't changed since the 50's.

Great information. I like the website too. However, I tried the formula's with info on my last 3 boats that were mono hulls under 20'. Formula did not come close to my actual plate ratings (fell quite short)...not sure why or if I missed something ( I am no math genius), but I am good with the motor ratings on the plates I have. Hopefully is works for others. I did it for a 1958 boat, and 1982 boat and a 1990 hull.

For the boat before 1972 it was not the law. It was an industry standard and some manufacturers ignored it. There was (and still is ) a test course method that sometimes resulted in a higher rating. But the most common error is in measurement, especially of the transom width. Add the rubrails. Plus it's maximum width, in other words the widest part. However on some boats that have a narrow transom it can be the widest part of the last quarter of the hull. Some builders added fins just to increase the width so it would increase the rating. On the newer boats just go with the capacity label.

When in question add another 100 hp.

Cal ... you are a funny guy. I think I messed up with width as some transom measurements I used were about a third of the actual boat width of back third.