Outboard Engine Sizing - Go big?

[JimS123]

Speed is a very poor indication of the overall performance of a boat.

Anyone who has battled against a stiff tide/current or pounded their way home in a following sea could care less about how fast the boat can go in ideal conditions.

Should a storm blow up out of nowhere, you can never go fast enough....

The speed / HP debate is really quite ludicrous. Yeah sure buy what you can afford.

Twice I OVER powered a boat and was sorry I did. I ended up either selling the boat or repowering with a smaller motor.

Twice in my 60 year boating lifetime did I ever encounter a dangerous following sea or with a storm that blew out of nowhere. In both cases my motor was bigger than I needed since I was having trouble even at near idle speed.

That won't happen to me any more. With a VHF and a cell phone I now know what's coming and I take the appropriate precautions way ahead of time.

 

[dingbat]

Twice in my 60 year boating lifetime did I ever encounter a dangerous following sea or with a storm that blew out of nowhere. In both cases my motor was bigger than I needed since I was having trouble even at near idle speed.

That won't happen to me any more. With a VHF and a cell phone I now know what's coming and I take the appropriate precautions way ahead of time.

Your very lucky. Cell coverage is spotty at best and the marine weather forecast for our area is a running joke.

To make matter worst, the topography of the area is conducive to the formation of frequent pulse type thunder storms. Very violent, short duration storms that blow up out of no where. Nine times of out ten you get no warning whatsoever from weather services.

For a brief (5-10 min) period, a pulse of severe weather is unleashed, but the damage is highly localized. The cells are only 3-5 miles in diameter.

The NWS forecaster may have little time to react to the swift growth of the cloud, and its equally swift collapse. A typical radar “sweep” or volume scan takes 8-10 minutes. Allowing for scan time, recognition of the event, and the time it takes to disseminate a warning to the public, the game is already over!

https://www.washingtonpost.com/news...-severe/?noredirect=on&utm_term=.28477e67660a

I have xm weather and 24 mile radar. Neither are much help in these situations. You keep an eye over your shoulder all summer and be prepared to hunker down if your can't elude the storm as it passes.

With the Bay's north-south orientation and it's prevailing southerly winds (sea breeze), a following sea is the norm on evenings with an incoming tide.

 

[ajgraz]

With a 17-18' boat, you really need to watch the engine's weight carefully for good performance (power to weight ratio). Yamaha and Merc 4 strokes take a huge jump when going over 115hp. Honda makes that same jump at 90hp. 2 strokes will have similar jumps but I don't have them in my head...

This is a very good point, given the size boat you are talking about.

My used 17' CC came originally with a 90hp carbed 2-stroke, though the hull is rated up to 135hp. IMO it was kind of a dog with only 90hp, and it blew a piston, in part I believe, because it spent its life being overworked outrunning storms on the Chesapeake.

When I brought it to CA and it came time to repower (with a CARB-friendly motor), we chose the 125 Opti because it was the largest of the 1.5L DFI line. It is quite a bit heavier than the motor it replaced, but at least my scuppers are still above water (after moving batteries forward, etc.). The 135 and bigger Opti is where they stepped up to the far heavier 2.5L...had we gotten the Opti 135, the scuppers would probably be submerged most of the time!

 

[FishingDisaster]

A big motor at light load is more fuel efficient tgan a small motor loaded heavily

This was my line of thinking but I couldn’t find anything to document that truth. So a 90 HP that frequently sees WOT is less efficient than a 115 HP that is usually at 3/4 throttle? And probably experience less engine wear?

 

[Gator5713]

This was my line of thinking but I couldn’t find anything to document that truth. So a 90 HP that frequently sees WOT is less efficient than a 115 HP that is usually at 3/4 throttle? And probably experience less engine wear?

The short answer to that is yes, but there are a lot of variables to that equation.
First understanding that HP is an equation of engine speed and torque [(RPM * T) / 5252=HP]
A smaller engine that spins faster can produce the same or more 'HP' than a big slow turning engine.
The second thing to consider is power to weight ratio and the law of diminishing returns.

 

[5150abf]

If you have 2 engines the same size and one runs at max or close most of the time and the other rarely goes over 3/4 throttle logic would be that the engine turned at less rpm would last longer.

If the boat is rated for a 150 I would think at least a 115-125.

Check the cost difference between the two and see if it is worth it.

Recently repowered and had the same conundrum, boat rated for 70, was wearing a 50.

Looked at 70-60-and a 50, I fish so only run full throttle at the beginning of the night then troll or run 10-15 mph between holes so for me the extra money for a bigger engine wasn't worth it so I went with another 50.

The boat actually runs 2-3 mph faster even though the new engine is 50 lbs heavier

I guess try to figure out how often you plan on running top speed and go from there.

Agreed, an engine does need to be ran hard now and then but certainly not all the time,