I've sailed small boats for many decades but am brand new to cruising in the "keeled realm" with my Mac 22. With my mast up, sails down, keel and rudder up (most of the way), I can do 6-6.3 kts with my Yamaha 5 hp 2 stroke if my GPS is correct. I've been reading these great forums and have come to some understanding of speed with displacement hulls but am uncertain if I can significantly increase my top motoring speed with a larger motor (for when those popup storms come up in the Keys!). Thanks for reading this and all the great postings. Stan.
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Probably as good as it gets
I can't say for sure because I don't know the length of the waterline on a Mac 22, but assuming its 18 feet, then you are already doing better than the theoretical maximum hull speed your boat should be capable of. All displacement hulls (those that push the water out of the way like a sailboat or trawler, as opposed to planing hulls that ride on top of the water) are limited in how fast they can go; they can't push over their own bow wave and can't outrun their stern wave. Someone very smart figured out a formula to calculate the supposed maximum speed based on the waterline length of any boat. The attached link is a calculator; plug in the length of your waterline and out pops your max speed. For an 18 foot waterline boat, the formula says you should be able to do no better than 5.69 knots. You will be able to find numerous stories of sailboats exceeding their hull speed, but mostly while surfing down waves. It doesn't sound to me like a bigger outboard is going to make any difference in speed for you.
I can't say for sure because I don't know the length of the waterline on a Mac 22, but assuming its 18 feet, then you are already doing better than the theoretical maximum hull speed your boat should be capable of. All displacement hulls (those that push the water out of the way like a sailboat or trawler, as opposed to planing hulls that ride on top of the water) are limited in how fast they can go; they can't push over their own bow wave and can't outrun their stern wave. Someone very smart figured out a formula to calculate the supposed maximum speed based on the waterline length of any boat. The attached link is a calculator; plug in the length of your waterline and out pops your max speed. For an 18 foot waterline boat, the formula says you should be able to do no better than 5.69 knots. You will be able to find numerous stories of sailboats exceeding their hull speed, but mostly while surfing down waves. It doesn't sound to me like a bigger outboard is going to make any difference in speed for you.
Probably not worth it, Stan-
Generally speaking, your top speed (hull speed) is based upon the length of your boat at the waterline (see, size does matter!). The formula is really just an approximation, but in reality, keel boats just can't go fast- the faster you go, the deeper the hull goes. A losing battle. Here's a real life scenerio for you- My buddy Reg bought a 23 Columbia with a 5hp. He upgraded to a 4 stroke, and since the 10 hp was just a few dollars more, he bought it. He was checking his speed, watching his handheld gps like a hawk when he suddenly found himself 2 inches deep in water- the hull had sank to the point where water was coming in through the scuppers (cockpit drains)! Anything past 1/2 throttle means wet feet. Into the wind, against the current, with a storm approaching the extra hp is great, but for most circumstances, it's wasted. cup
Generally speaking, your top speed (hull speed) is based upon the length of your boat at the waterline (see, size does matter!). The formula is really just an approximation, but in reality, keel boats just can't go fast- the faster you go, the deeper the hull goes. A losing battle. Here's a real life scenerio for you- My buddy Reg bought a 23 Columbia with a 5hp. He upgraded to a 4 stroke, and since the 10 hp was just a few dollars more, he bought it. He was checking his speed, watching his handheld gps like a hawk when he suddenly found himself 2 inches deep in water- the hull had sank to the point where water was coming in through the scuppers (cockpit drains)! Anything past 1/2 throttle means wet feet. Into the wind, against the current, with a storm approaching the extra hp is great, but for most circumstances, it's wasted. cup
Perfectly Adequate
Stan, Welcome to the world of displacement hulls. The formula for determining the theoretical maximum speed through the water for a displacement hull has traditionally been expressed as 1.34 times the square root of the length of the hull at the waterline (LWL). Your Mac has a length on deck (LOD) of approx. 22 ft. The LWL is likely to less than 20 ft. But being generous, and using 20 ft. as your LWL, the formula predicts your max. speed right at 6kts. Seems your 5hp motor is doing a fine job. Caveat: your GPS is measuring Speed Over Ground, not speed through the water. If there is a significant current running, some adjustment must be made to get an accurate result. Once the hull reaches its hull speed, it takes a LOT of additional power to produce a small increase in speed, on a non-planing hull. I'd keep the 5ph motor, and note how it performs when you are trying to drive into a stiff breeze and/or large swells, where a little extra horsepower may make the difference between making decent headway or not. Fair Winds, Jeff
Stan, Welcome to the world of displacement hulls. The formula for determining the theoretical maximum speed through the water for a displacement hull has traditionally been expressed as 1.34 times the square root of the length of the hull at the waterline (LWL). Your Mac has a length on deck (LOD) of approx. 22 ft. The LWL is likely to less than 20 ft. But being generous, and using 20 ft. as your LWL, the formula predicts your max. speed right at 6kts. Seems your 5hp motor is doing a fine job. Caveat: your GPS is measuring Speed Over Ground, not speed through the water. If there is a significant current running, some adjustment must be made to get an accurate result. Once the hull reaches its hull speed, it takes a LOT of additional power to produce a small increase in speed, on a non-planing hull. I'd keep the 5ph motor, and note how it performs when you are trying to drive into a stiff breeze and/or large swells, where a little extra horsepower may make the difference between making decent headway or not. Fair Winds, Jeff
I agree yours is adequate and add
that when you jump from 5 to 8 or 10 horse, you usually go from 1 to 2 cylnder. This greatly increases overall engine mass- not just the powerhead but also the lower unit, prop and clamps. It is a whole level harder to install by yourself. I have sailed an O'Day 23 with both a 5 and a 10 horse four stroke. The boat is happier with the 5.
that when you jump from 5 to 8 or 10 horse, you usually go from 1 to 2 cylnder. This greatly increases overall engine mass- not just the powerhead but also the lower unit, prop and clamps. It is a whole level harder to install by yourself. I have sailed an O'Day 23 with both a 5 and a 10 horse four stroke. The boat is happier with the 5.
I am running a 6hp 4stroke Tohatsu
on my 222, I can reach hull speed at 1/2 throttle, the transom may not support a bigger motor without substantial reinforcement. Since you don't want to trailer it with the motor attached, getting it on and off is a concern. Tim
on my 222, I can reach hull speed at 1/2 throttle, the transom may not support a bigger motor without substantial reinforcement. Since you don't want to trailer it with the motor attached, getting it on and off is a concern. Tim
GPS
As Anchor Down noted, GPS gives ground speed...that has nothing to do with speed thru the water or hull speed. Wind, tide, etc all affect GPS speed...use your knot meter for hull speed.
As Anchor Down noted, GPS gives ground speed...that has nothing to do with speed thru the water or hull speed. Wind, tide, etc all affect GPS speed...use your knot meter for hull speed.
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