Speed vs stability?

[ontherocks83]

Alright I want to start off again by mentioning that I have been sailing for 25 years and owned a jet ski for 10 but this is my first year ever owning a pointy go fast boat. Ok maybe mine isn't all that fast but compared to my sail boat its a rocket. So please bear with me on some of my questions

So I have a 1989 Strobe 201 with a 350 magnum. I currently top out at about 55-57 mph at 4500 rpm. If I trim up any further it starts to chine walk and the label on the engine says max rpm is 4500 so I figure it is propped correctly and that is about the best I will get out of her. The prop is stainless and damaged (purchased that way) so I am hoping / hypothesising that once I get the prop rebuilt I might get 60 out of it or atleast a small increase in speed.

I see alot of boats on here that are a similar length that get 80+ mph. Some in the 70's and some over 100. I believe most are outboards but I could be wrong. I know outboards tend to have a better top end in general and no I am not expecting to ever get my boat that fast.

My question is: How stable are some of these smaller boats at those speeds? Does the water have to be mirror smooth to run like that? How do those smaller boats that run so fast handle at 50 mph in rougher waters? What is the benefit of having my boat with an I/O? Is mine more stable in rough waters or is it just a slower boat?

Basically besides top speed What are the big differences between my boat and some of these crazy fast boats (under 25 feet). Is it interior room, towing, stability in rough waters?

Is it comparable to a mustang vs a sport bike where the mustang is fast enough, can hold 5 people and still be used safely in the rain, where the sport bike is blindingly fast but if the conditions arent perfect then they dont go out. Or is it they just have better mustangs?

Thanks all

 

[jazzy]

Power to weight ratio, and setup are big wins for the ob boats in that size. You can trim your out drive out to lift your bow but only so much. The faster ob boats are on hydraulic brackets( don't remember what you call them) so they are set back much further and can also raise the prop higher than you can without changing the angle of attack. In essence they will run faster and flatter, but eventually everything starts to chine that's where the driver comes in. There's a lot to the setup, but those are the basics. I'm sure somebody will chime in with more detailed reasons .

 

[ontherocks83]

Power to weight ratio, and setup are big wins for the ob boats in that size. You can trim your out drive out to lift your bow but only so much. The faster ob boats are on hydraulic brackets( don't remember what you call them) so they are set back much further and can also raise the prop higher than you can without changing the angle of attack. In essence they will run faster and flatter, but eventually everything starts to chine that's where the driver comes in. There's a lot to the setup, but those are the basics. I'm sure somebody will chime in with more detailed reasons .

The jack plates make sense. What about stability or sea worthiness in choppy waters as opposed to those boats on flat calm?

 

[atc250r]

Depends on how big the water is but generally the extra weight of a big, heavy, cast iron V8 car engine on the back makes an I/O better in rough water but in a 1 to 2 footers my outboard runs faster than on smooth water as it gets up on top of the chop.

 

[SaskExciter]

Hey I was just reading your post and figured I'd chime in with my .02 $ I'm copying much of this from an email I just wrote a buddy on the same topic. I'm speaking generally here, and I could totally be misinformed about some of these points but all of my research and seat time backs up what I'm saying. May be more than you care to read but may be interesting to you so here goes!

Hp for hp outboards are almost always more efficiently laid out for top speed. Imagine the back of your boat, or where the hull is deepest in the water at rest, as a fulcrum point upon which the various forces are acting to either bring the nose down and have greater wet surface (= friction and slower) or less of the hull in the water (= aired out and faster). Here are Some of the major forces at work and how outboards vs I/o's compare: 1) Over all weight - more weight means more water needs to be displaced to keep you afloat or more surface area of hull must contact the water at a given speed to carry the boat at a given speed, meaning more weight needs more power to reach or maintain that given speed. Outboards are generally lighter. 2) direction and magnitude of thrust from the prop - the thrust from the prop is, in both ob's and io's behind the fulcrum of the boat so trimming either up from neutral will direct the prop such that the "water shooting out the back" will be aimed up a bit from parallel to the direction of travel along the surface. This will push down on the back of the boat which levers the bow up out of the water, wetting less surface and making you go faster. props on outboard engines can be set back further from the fulcrum so that a smaller change in direction of thrust will produce the same or more lift. This is more efficient as it preserves a higher % of thrust along your direction of travel for speed for a given amount of bow lift. 3) weight and length of the lever arm on either side of the fulcrum - weight behind the fulcrum point will act to lift the bow even without trim. Outboard motors are behind this fulcrum and setback can increase this effect. Io's are fixed nearly over top of the pivot point. This sort of has an outboard's weight working for them (sort of!) and helps the outboard preserve still more power for forward speed. The io's need don't have this benefit. On the other end, you need something to counter act the weight of the motor and such but light is better (for speed) so long light weight nose is ideal for crazy speeds. That is why crazy fast v-hulls like hydrostreams have those menacing looking long light noses. So outboards are just faster than io's power for power BUT!! Everything is a trade off and for most of these same reasons an io is generally more stable:

- weight closer to the fulcrum, and lower in the boat is generally more stable at speed. Outboards are lifted up, and back. Io's are low and over top of fulcrum = more stable and easier to balance at speed. Io's have More weight in general and usually hulls with more dead rise, both traits are better suited to bigger water.

An additional advantage for outboard speed is a jack plate lifting the lower unit out of the water reducing the amount cutting through the water and reducing drag. Io's are stuck deep in the water. An advantage for io's though, I would say is that it's easier to make power adding mods to your 350 than to my 2stroke outboard! So if you are making way more hp, all this power for power stuff gets put into prospective in s hurry!
Now both io's and outboards can be set up to optimize what they're working with for speed and or hole shot and or rough water abilities. Different props will carry a bow better helping with weight and lack of trimming ability. Some props work better deeper in the water on io's. With proper balancing, tight steering, and seat time a little outboard can handle some decent chop at speed too.

For example I have my 18'6 exciter with 175 outboard hp set up so that in ideal conditions I have hit 64mph GPS, and can drive it at 60 through 1' chop with one hand just like it was doing 40, but it took some time to get it there. Same boat same motor same steering components two years ago topped out at 60 GPS with slower hole shot and was crazy hard to handle at that speed even in near flat water. I'm not the guy to help set up your io but I'm sure many folks on here will be great help! Only problem with dialling in your set up is that it makes you want more power! Welcome to the sport!

 

[merc245]

Hey I was just reading your post and figured I'd chime in with my .02 $ I'm copying much of this from an email I just wrote a buddy on the same topic. I'm speaking generally here, and I could totally be misinformed about some of these points but all of my research and seat time backs up what I'm saying. May be more than you care to read but may be interesting to you so here goes!

Hp for hp outboards are almost always more efficiently laid out for top speed. Imagine the back of your boat, or where the hull is deepest in the water at rest, as a fulcrum point upon which the various forces are acting to either bring the nose down and have greater wet surface (= friction and slower) or less of the hull in the water (= aired out and faster). Here are Some of the major forces at work and how outboards vs I/o's compare: 1) Over all weight - more weight means more water needs to be displaced to keep you afloat or more surface area of hull must contact the water at a given speed to carry the boat at a given speed, meaning more weight needs more power to reach or maintain that given speed. Outboards are generally lighter. 2) direction and magnitude of thrust from the prop - the thrust from the prop is, in both ob's and io's behind the fulcrum of the boat so trimming either up from neutral will direct the prop such that the "water shooting out the back" will be aimed up a bit from parallel to the direction of travel along the surface. This will push down on the back of the boat which levers the bow up out of the water, wetting less surface and making you go faster. props on outboard engines can be set back further from the fulcrum so that a smaller change in direction of thrust will produce the same or more lift. This is more efficient as it preserves a higher % of thrust along your direction of travel for speed for a given amount of bow lift. 3) weight and length of the lever arm on either side of the fulcrum - weight behind the fulcrum point will act to lift the bow even without trim. Outboard motors are behind this fulcrum and setback can increase this effect. Io's are fixed nearly over top of the pivot point. This sort of has an outboard's weight working for them (sort of!) and helps the outboard preserve still more power for forward speed. The io's need don't have this benefit. On the other end, you need something to counter act the weight of the motor and such but light is better (for speed) so long light weight nose is ideal for crazy speeds. That is why crazy fast v-hulls like hydrostreams have those menacing looking long light noses. So outboards are just faster than io's power for power BUT!! Everything is a trade off and for most of these same reasons an io is generally more stable:

- weight closer to the fulcrum, and lower in the boat is generally more stable at speed. Outboards are lifted up, and back. Io's are low and over top of fulcrum = more stable and easier to balance at speed. Io's have More weight in general and usually hulls with more dead rise, both traits are better suited to bigger water.

An additional advantage for outboard speed is a jack plate lifting the lower unit out of the water reducing the amount cutting through the water and reducing drag. Io's are stuck deep in the water. An advantage for io's though, I would say is that it's easier to make power adding mods to your 350 than to my 2stroke outboard! So if you are making way more hp, all this power for power stuff gets put into prospective in s hurry!
Now both io's and outboards can be set up to optimize what they're working with for speed and or hole shot and or rough water abilities. Different props will carry a bow better helping with weight and lack of trimming ability. Some props work better deeper in the water on io's. With proper balancing, tight steering, and seat time a little outboard can handle some decent chop at speed too.

For example I have my 18'6 exciter with 175 outboard hp set up so that in ideal conditions I have hit 64mph GPS, and can drive it at 60 through 1' chop with one hand just like it was doing 40, but it took some time to get it there. Same boat same motor same steering components two years ago topped out at 60 GPS with slower hole shot and was crazy hard to handle at that speed even in near flat water. I'm not the guy to help set up your io but I'm sure many folks on here will be great help! Only problem with dialling in your set up is that it makes you want more power! Welcome to the sport!

Wait, What?

 

[Jimway]

See, this is kinda what I was talkin about. Pretty soon though, I'm gonna throw in some jet thrust stuff.