TOPIC: Determining engine size - max hp

I think I saw a hp value of 62hp for my 58 Glass Magic Playmaster.
I have to build the transom. Can I make it stronger to hold a 80hp? Or is there other design limitations I should be aware of?

Also for these old boat with out coast guard plates. I was thinking about making my own. What do others do?

Thanks, Dave

If you run out of fresh underwear, it's too big.


Most of the boats we play with from the fifties are not designed to go faster than 40-45 mph. Many of them become unstable at speeds higher than that and exhibit dangerous handling characteristics - porpoising, rolling or hooking in a turn, digging in the bow when coming up behind a following wave...

In addition, many of the hulls are not built strong enough to bear the weight of a motor bigger than they were originally rated for.

Hey Bill,

I used to use that saying when it came to cars and engine size. Thanks for the laugh!

There is an old chart that gives a guideline for this. I think it is somewhere on the site, but I can't find where. I have a hard copy here. You calculate the product of the overall length and the stern width, minus fins and then compare it to the graph to determine max HP.

2-19-01
PLANING BOAT SPEED CHARTS

"How fast will it go?" is a frequently asked question. There isn't a simple answer, as many factors must be considered. However Chart 1 will provide a rough estimate of the potential speeds possible for a typical planing boat.
Chart 1
pounds per horsepower chart

The chart is based on pounds per horsepower, a factor obtained by dividing the gross weight by the shaft horsepower. Note that "gross weight" is the total weight of the boat as it would be fully equipped with passenger(s) on board ready to go. Don't overlook the fuel (6.2 lbs. per gallon) and ANY extra gear carried. After figuring the total weights add a 10% safety factor.

To use the chart, locate the gross weight divided by the shaft horsepower factor along the bottom of the chart. Go upwards from this line to the "average planing hull" curve and read the speed in mph.

Note how important the weight factor is in a planing boat, and the smaller the boat the more important it becomes.
Chart 2
Weight of typical outboard

As an assist in determining gross weight Chart 2 indicates the weight of a typical outboard motor based on the horsepower. The weights are an average taken from several outboard motor manufactures and included four cycle types.

The 14' Larson Falls Flyer claimed it could take a 50 HP outboard. Now as we all know, a 50 in the late 50's was a big V4. If it can handle a V4, for weight, does it matter the Horse power? If you jump a wave made by a giant cruiser, and land squarely on the second wave, the dash board is still going to splinter and fall in your lap, the windshield will break off, and the back of the seat brace will split and fall away, whether you are running modern 2 cylinder 50hp or V4 115.

The only difference is the 2 cylinder engine is easier for your buddy in the back seat to grab hold of to steer. (Because the steering wheel and pulleys are now in a pile on the floor.)

(don't ask me how I know this)

jepstr67, wow did that happen to you? must have been a rush. i would like to look at your thunderhawk, is it in st.cloud or up north? frog

Great charts, thank you. Top speed is not my goal. I think my question was better answered with estimated weight of engines. I have no knowledge base on any of this. Including weights of engines, hp ratings, and construction of hulls and transoms.

Main reason for question is due to tiny budget for this project, I am looking for a $600 dollar type boat that runs. I found one with a Mercury 80 with that gorgeous ribbed chrome?metal cover.

BETTER QUESTION what is weight of a 59 ish Mercury 80?

Winter Sucks, Dave

I was out on Lake Minnetonka and there was no wind. When there is no wind on a lake where there are huge cruisers, there are waves going all different directions which stay large because there is no natural wind chop to knowck the power out of them. I did not see one of these such cruiser waves, went air born off the first one and landed squarely, belly flop style, on the second one. I have a 1979 Evinriude 55 HP on my Flyer.

Yes. The T-Hawk is in St.Cloud.

MadProps wrote:

2-19-01
PLANING BOAT SPEED CHARTS

"How fast will it go?" is a frequently asked question. There isn't a simple answer, as many factors must be considered. However Chart 1 will provide a rough estimate of the potential speeds possible for a typical planing boat.
Chart 1
pounds per horsepower chart

The chart is based on pounds per horsepower, a factor obtained by dividing the gross weight by the shaft horsepower. Note that "gross weight" is the total weight of the boat as it would be fully equipped with passenger(s) on board ready to go. Don't overlook the fuel (6.2 lbs. per gallon) and ANY extra gear carried. After figuring the total weights add a 10% safety factor.

To use the chart, locate the gross weight divided by the shaft horsepower factor along the bottom of the chart. Go upwards from this line to the "average planing hull" curve and read the speed in mph.

Note how important the weight factor is in a planing boat, and the smaller the boat the more important it becomes.
Chart 2
Weight of typical outboard

As an assist in determining gross weight Chart 2 indicates the weight of a typical outboard motor based on the horsepower. The weights are an average taken from several outboard motor manufactures and included four cycle types

But for what model year? As I said, when looking at the basic OMC V4 it had a huge range of horse powers over many years where weight changed very little. Even between 59 and 65 V4s went from 50 to 90 HP with basically the same hardware.

It’s pourin’ rain, flood warnings here as the remains of Rina go by so I thought I’d look into it for something to do. I don’t know your boat first hand so I looked it up in the FG library, neat little boat. The Playmaster in 1957 was rated for 40 hp, in 1958 it was rated for 50 hp, in 1959 it was rated for 60 hp.
Boat weight
1957…..325 lbs
1958…..425 lbs
1959…..425lbs

Looking up in my Merc books what would have been recommended for Merc power.
1957…..Mark 58E Thunderbolt, 40 hp, 120 lbs
1958…..Mark 58E Super Thunderbolt, 45hp, 131 lbs
1959…..Mark 75A Marathon “Six”, 60 hp, 185 lbs (there was also a 1957-58 Mark 75E Marathon “Six” 60 hp, 168 lbs)

When rigging a motor on a boat, the weight of the motor is my primary concern. The transom supports the weight of it and other pressures on the motor, and when installed properly the transom will transfer those forces up the hull. The weight the transom feels can very as the motor bounces (trailer is more than in water), or the motor gets used for a step or seat, motor/prop torque, or the resistance of the water. The faster you go the more water pressure on the lower unit of the motor and thus to the transom.

Weather the hull can handle the speed? Talk to some one that tried it, or you gotta’ try yourself. Start slow and work your way up, checking regularly for cracking. The hull will get squirrelly if you are going to fast, just back off a little. To me it’s a bonus to go full speed and not run the motor WOT.

jepstr…how fast were you going? I did that on a kneeboard once, about 35mph. The second wave was quite an impact, it launched me 10’ in the air. Held on and crashed the next landing. The board went under water and I face planted. Saw a boat pancake on the Mississippi once. He was jumping wakes and landed on a floating submerged tree.